Microbiology MCQs Chapter 13 (Questions 601–650) with Detailed Answers & Explanations

Explanation: Serological reactions are diagnostic methods used to identify the interaction between antigens and antibodies. They are used to identify unknown antigens (using known antibodies) or to detect the presence of specific antibodies in a patient’s serum (using known antigens).

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Why other options are incorrect:

• • Detection of antigens: While true, it is incomplete because serology also detects antibodies.
• • Detection of antibodies: While true, it is incomplete because serology also detects antigens.
• • None of these: This is incorrect as the primary purpose of serology is the detection of antigens and antibodies.

Explanation: Immunoelectrophoresis is a laboratory technique that combines electrophoresis (to separate antigens based on their electric charge and size) with immunodiffusion (to identify them using specific antibodies).

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Why other options are incorrect:

• • Flocculation: This is a type of precipitation reaction where the antigen-antibody complex forms visible flakes or “floccules” in suspension, rather than being used for separation.
• • Agglutination: This involves the clumping of particulate antigens (like bacteria or red blood cells) with antibodies, used primarily for detection or titration, not for separating different antigens.
• • None of these: This is incorrect because Immunoelectrophoresis is a standard method for antigen separation.

Explanation: Agglutination is the visible clumping of particulate antigens (such as bacteria or red blood cells) when they react with specific antibodies (agglutinins) in the presence of electrolytes at an optimal pH and temperature.

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Why other options are incorrect:

• • Precipitation: This involves the reaction of soluble antigens with antibodies to form an insoluble visible complex, rather than the clumping of particles.
• • Electrophoresis: This is a technique used to separate molecules (like proteins or DNA) based on their size and electrical charge using an electric field.
• • CIE: Counter Immunoelectrophoresis (CIE) is a procedure where antigen and antibody are moved toward each other in an electric field to form a precipitation line; it is not a general clumping reaction.

Explanation: Neutralisation is a serological reaction where the biological effects of toxins or viruses are “neutralized” by specific antibodies. For example, an antitoxin can neutralize a bacterial toxin, or antiviral antibodies can block a virus’s ability to infect a cell.

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Why other options are incorrect:

• • Precipitation: This reaction involves soluble antigens and antibodies forming an insoluble complex, typically used for protein analysis rather than biological activity.
• • Agglutination: This involves the clumping of particulate antigens (like bacteria). While it can detect the presence of a virus, it doesn’t measure the neutralizing capacity/effect of the antibody against the toxin.
• • None of these: This is incorrect because Neutralisation is a standard and vital method for these detections.

Explanation: Exotoxins are proteins secreted by bacteria. Proteins are the most potent antigens because of their high molecular weight and complex tertiary structure, which allows the immune system to recognize them more easily.

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Why other options are incorrect:

• • Endotoxins: These are lipopolysaccharides (LPS). While they trigger a strong inflammatory response, they are generally less immunogenic/antigenic than pure protein exotoxins.
• • Viruses: While viruses are highly antigenic, the question usually seeks to compare the chemical nature of substances. Among biological molecules, protein-based exotoxins are the benchmark for high antigenicity.
• • All of these: Since exotoxins have a significantly higher level of antigenicity compared to endotoxins, this option is incorrect.

Explanation: The Schick test is a skin test used to determine whether or not a person is susceptible to Diphtheria. It involves injecting a small amount of diluted diphtheria toxin intradermally to check for the presence of neutralizing antibodies.

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Why other options are incorrect:

• • T.B. (Tuberculosis): The Mantoux test (Tuberculin skin test) is used for the detection of TB, not the Schick test.
• • Cholera: Diagnosis is usually confirmed through stool culture or rapid diagnostic tests (RDT).
• • Typhoid: The Widal test is the classic serological test used for the diagnosis of Typhoid fever.

Explanation: The complement system is a part of the immune system that enhances (complements) the ability of antibodies and phagocytic cells. Its biological functions include Haemolysis (direct lysis of foreign cells via the MAC complex) and Phagocytosis (facilitated through opsonization by C3b).

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Why other options are incorrect:

• • Haemolysis: While this is a function, it is not the only secondary function of the complement system.
• • Phagocytosis: Complements aid phagocytosis through opsonization, but focusing only on this ignores the lytic capabilities (haemolysis).
• • None of these: This is incorrect because both haemolysis and opsonization (leading to phagocytosis) are core biological activities of the complement cascade.

Explanation: ELISA (Enzyme-Linked Immunosorbent Assay) is highly effective because of its high sensitivity and specificity. It is less time-consuming due to automation and can process 96 or more samples simultaneously using microtiter plates, making it the standard for high-throughput screening.

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Why other options are incorrect:

• • CFT (Complement Fixation Test): This is a complex, labor-intensive, and time-consuming procedure that requires many reagents and careful standardization.
• • Neutralization: These tests often require live cell cultures and several days of incubation, making them very slow for large-scale testing.
• • Agglutination: While fast and simple, it is generally less sensitive than ELISA and harder to automate for the simultaneous detection of hundreds of quantitative samples.

Explanation: B-cells (B-lymphocytes) are the key components of Humoral immunity. When they encounter an antigen, they differentiate into plasma cells that produce and secrete antibodies to neutralize pathogens in the “humors” (body fluids).

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Why other options are incorrect:

• • Cell-mediated immunity: This is primarily the function of T-cells (T-lymphocytes), which destroy infected host cells directly rather than using antibodies.
• • Active immunity: This refers to the immunity developed after exposure to an antigen (via infection or vaccine). While B-cells are part of this process, they are specifically categorized by their mechanism as “humoral.”
• • Passive immunity: This involves receiving pre-formed antibodies (like from breast milk or an injection). In this case, the recipient’s own B-cells are not actively involved in producing the immunity.

Explanation: Innate immunity is the first line of defense that is present from birth. It is non-specific because it provides a general defense against all pathogens (using physical barriers, phagocytes, etc.) rather than targeting a particular strain or type of microbe.

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Why other options are incorrect:

• • Specific: This describes Adaptive (Acquired) immunity, which uses specialized cells like B-cells and T-cells to target specific antigens.
• • Active: Active immunity refers to the immune response triggered by exposure to a pathogen or vaccine; it is a subset of adaptive immunity.
• • Passive: Passive immunity involves the transfer of pre-made antibodies (like through the placenta or injection) and is also a subset of adaptive immunity.

Explanation: Innate immunity utilizes both mechanical barriers (like the skin, mucous membranes, and cilia) and chemical barriers (like stomach acid, lysozymes in tears/saliva, and fatty acids on the skin) to prevent the entry and growth of pathogens.

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Why other options are incorrect:

• • Mechanical barriers: These are a major part of innate immunity (e.g., skin), but they do not act alone; chemical defenses are equally vital.
• • Chemical barriers: While essential (e.g., low pH in the stomach), they represent only one facet of the innate immune system.
• • None of these: This is incorrect because mechanical and chemical barriers are the primary first-line defenses of the innate immune system.

Explanation: Acquired (Adaptive) immunity is the immunity a person develops during their lifetime. It is highly diverse and can be classified as Natural (via infection or colostrum), Artificial (via vaccines or pre-formed antibodies), and can further be divided into Active and Passive types.

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Why other options are incorrect:

• • Natural: This is only one way acquired immunity is obtained (e.g., following a natural infection).
• • Artificial: This is only one way acquired immunity is obtained (e.g., through medical intervention like vaccination).
• • Active & Passive: These represent the two functional mechanisms of acquired immunity, but they don’t cover the full classification on their own.

Explanation: Acquired immunity is developed in two ways: Natural means (such as recovering from an infection or receiving antibodies from a mother) and Artificial means (such as receiving a vaccine or an injection of immune globulin).

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Why other options are incorrect:

• • Natural means: While correct, it is incomplete as it ignores modern medical interventions like vaccination.
• • Artificial means: While correct, it is incomplete as it ignores immunity gained through natural exposure to pathogens.
• • None of these: This is incorrect because natural exposure and medical intervention are the two fundamental ways we acquire specific immunity.

Explanation: Immediate hypersensitivity reactions are classified as Type-I hypersensitivity. They occur within minutes of exposure to an allergen and are mediated by IgE antibodies and mast cells, resulting in the release of histamine and other inflammatory mediators.

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Why other options are incorrect:

• • Type-II: Antibody-mediated cytotoxic hypersensitivity involving IgG or IgM antibodies.
• • Type-III: Immune complex-mediated hypersensitivity caused by antigen-antibody complex deposition.
• • All of these: Only Type-I is classified as immediate hypersensitivity.

Explanation: Immediate (Type I) hypersensitivity reactions are mediated by mast cells. These cells possess IgE antibodies on their surface and release histamine and other inflammatory mediators upon exposure to an allergen, causing allergic reactions such as asthma, hay fever, and anaphylaxis.

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Why other options are incorrect:

• • T-cells: Primarily involved in Type IV (delayed) hypersensitivity reactions.
• • β-cells: Pancreatic cells involved in insulin production, not hypersensitivity reactions.
• • Macrophages: Function in phagocytosis and chronic inflammation but are not the primary mediators of immediate hypersensitivity.

Explanation: Cell-mediated immunity is primarily mediated by T-lymphocytes rather than antibodies. Examples include the tuberculin reaction, contact dermatitis, and granulomatous hypersensitivity, all of which are forms of delayed-type (Type IV) hypersensitivity reactions.

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Why other options are incorrect:

• • Tuberculin type: This is a correct example of cell-mediated immunity, but not the only one.
• • Contact dermatitis: Also mediated by T-cells and represents delayed hypersensitivity.
• • Granulomatous: Another form of cell-mediated immune response involving activated macrophages and T-cells.

Explanation: The Mantoux test (often referred to as the Mountax reaction in some contexts) is a skin test used to detect Tuberculosis (T.B.). It involves the intradermal injection of Purified Protein Derivative (PPD). A positive result is indicated by a hard, raised area (induration) at the site after 48-72 hours.

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Why other options are incorrect:

• • Diphtheria: This is detected using the Schick test, which determines susceptibility to the diphtheria toxin.
• • Cholera: This is typically diagnosed via stool culture or rapid diagnostic tests (RDTs) to identify Vibrio cholerae.
• • None of these: This is incorrect because the test is the primary screening tool for tuberculosis infection.

Explanation: According to the Clonal Selection Theory, a single B-cell is programmed to recognize one specific epitope. When that B-cell is activated and clones itself into plasma cells, all the antibodies produced by that lineage have similar specificity (the same antigen-binding site).

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Why other options are incorrect:

• • Different specificities: This would contradict the basic principle of the immune system’s targeted response. Different specificities come from different B-cell lineages, not the same one.
• • Similar size: While antibodies of the same class (e.g., IgG) have similar sizes, this is a physical property and not the defining functional characteristic of antibodies produced by a specific B-cell.
• • None of these: This is incorrect because “similar specificity” is the fundamental biological rule for B-cell antibody production.

Explanation: The hybridoma technique is used to produce monoclonal antibodies. It involves the fusion of a Spleen cell (specifically a B-lymphocyte that produces a specific antibody) and a Myeloma cell (a cancerous plasma cell that is immortal). The resulting “hybridoma” cell possesses both the antibody-producing ability of the B-cell and the immortality of the myeloma cell.

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Why other options are incorrect:

• • Spleen cell – Spleen cell: Fusing two normal spleen cells would result in a cell that produces antibodies but has a limited lifespan and will die in culture.
• • Myeloma cell – Myeloma cell: Fusing two myeloma cells would result in an immortal cell line, but it would not produce the specific antibody desired for the experiment.
• • None of these: This is incorrect as the standard procedure for monoclonal antibody production specifically requires the spleen-myeloma fusion.

Explanation: The Anthrax vaccine was historically developed by Louis Pasteur using attenuated bacilli (weakened by growing them at a higher temperature). Modern veterinary vaccines, like the Sterne strain, still use live attenuated spores that lack the protective capsule required to cause disease.

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Why other options are incorrect:

• • Killing the bacilli: While killed vaccines exist for some diseases, the primary immune response for Anthrax is most effectively triggered by live attenuated strains or cell-free filtrates (toxoid-based).
• • Live bacilli: Using fully virulent live bacilli would cause the disease (Anthrax) rather than preventing it. The bacteria must be attenuated (weakened) to be safe for vaccination.
• • None of these: This is incorrect as attenuation is a standard method for producing the Anthrax vaccine.

Explanation: The Sabin vaccine (also known as OPV – Oral Polio Vaccine) consists of live-attenuated (weakened) polioviruses. Because it is administered orally, it induces both systemic immunity and local mucosal immunity in the gut.

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Why other options are incorrect:

• • BCG: The Bacillus Calmette–Guérin vaccine is used to protect against Tuberculosis, not Polio.
• • Measles vaccine: This is a specific vaccine used to prevent Measles (often given as part of the MMR shot).
• • TAB vaccine: This is a combined vaccine used to provide protection against Typhoid and Paratyphoid A and B.

Note: Do not confuse the Sabin vaccine with the Salk vaccine; the Salk vaccine (IPV) is an inactivated (killed) vaccine given by injection.

Explanation: The Salk vaccine (also known as IPV – Inactivated Poliovirus Vaccine) was developed by Jonas Salk. It is prepared using killed polioviruses and is administered via injection.

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Why other options are incorrect:

• • Sabin vaccine: This is the Live-Attenuated oral vaccine (OPV), not the killed version.
• • BCG: This vaccine is used specifically for the prevention of Tuberculosis.
• • TAB: This is a killed vaccine used against Typhoid and Paratyphoid fever, not Polio.

Explanation: According to the World Health Organization (WHO) and many national immunization schedules (such as in India), the first dose of the Measles vaccine is typically administered at 9 months of age. The second dose is usually given later, and catch-up vaccinations can be administered up to 10-15 years of age if missed earlier.

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Why other options are incorrect:

• • 1 year: While some developed countries start MMR at 12–15 months, 9 months is the standard globally to prevent early childhood mortality from measles.
• • 7 months: This is generally too early because maternal antibodies against measles are often still present in the infant’s blood, which can neutralize the vaccine and make it ineffective.
• • None of these: This is incorrect as “9 months to 10 years” covers the standard primary and catch-up window for the vaccine.

Explanation: The traditional whole-cell pertussis vaccine (wP) is prepared by growing Bordetella pertussis and then inactivating (killing) the bacteria. Formalin (formaldehyde) is the chemical commonly used to kill the bacilli while ensuring the antigens remain intact to stimulate an immune response.

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Why other options are incorrect:

• • Heat killed: Although heat can be used to kill bacteria, chemical inactivation with formalin is preferred for this vaccine to better preserve the immunogenic properties of the bacterial surface.
• • Attenuated / Live: There is currently no widely used live-attenuated vaccine for Pertussis in standard immunization schedules; the vaccines are either whole-cell killed or acellular (containing only purified protein parts).

Explanation: DPT is known as a Triple vaccine because it provides immunity against three different infectious diseases: Diphtheria, Pertussis (Whooping Cough), and Tetanus.

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Why other options are incorrect:

• • Double vaccine: This refers to vaccines like DT (Diphtheria and Tetanus only), which lacks the Pertussis component.
• • Tetanus toxoid: This is a single vaccine (monovalent) that only protects against Tetanus. While it is part of DPT, it does not define the whole DPT combination.
• • All of these: Since DPT is specifically defined by its triple-action nature, it cannot be classified as a double or single vaccine simultaneously.

Explanation: DPT vaccine is a combined vaccine that provides protection against Diphtheria, Pertussis (whooping cough), and Tetanus. Therefore, all the listed diseases/components are included in the DPT vaccine.

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Why other options are incorrect:

• • Diphtheria: Covered by DPT, but it is only one component of the vaccine.
• • Pertussis vaccine: Also included in DPT, but not the complete answer.
• • Tetanus toxoid: Another component of DPT, but not the only one.

Explanation: A Mixed vaccine (also known as a combination vaccine) consists of two or more different types of immunizing agents (antigens) combined in a single injection to provide protection against multiple diseases (e.g., DPT or MMR).

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Why other options are incorrect:

• • Cellular vaccine: These contain whole cells of the infectious agent (either killed or live-attenuated) rather than just a specific part.
• • Recombinant vaccine: These are produced using genetic engineering technology, where DNA encoding an antigen is inserted into bacterial or yeast cells.
• • Toxoid vaccine: These are made from toxins that have been secreted by the bacteria and rendered harmless (e.g., Tetanus toxoid).

Explanation: A Mixed vaccine (also known as a combination vaccine) is one that contains more than one type of antigen or immunizing agent to protect against multiple diseases with a single shot. Examples include the DPT (Diphtheria, Pertussis, and Tetanus) and MMR (Measles, Mumps, and Rubella) vaccines.

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Why other options are incorrect:

• • Cellular vaccine: These contain the whole cell of the microorganism, which has been killed or weakened, rather than a mix of different agents.
• • Recombinant vaccine: These are produced using genetic engineering where a specific piece of DNA is inserted into cells to trigger an immune response.
• • Toxoid vaccine: These are specifically made from inactivated toxins (poisonous byproducts) produced by certain bacteria.

Explanation: Inactivated vaccines are produced by killing the disease-causing microbe with chemicals, heat, or radiation. Because the microbes are dead, they cannot cause the disease, but they still trigger an immune response.

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Why other options are incorrect:

• • Attenuated vaccines: These use a weakened (live) form of the germ that causes a disease, rather than a killed one.
• • Autogenous vaccine: These are custom vaccines prepared from pathogens isolated from a specific patient or herd to be used on that same patient or herd.
• • None of these: This is incorrect because option ‘a’ accurately describes vaccines made from killed microbes.

Explanation: Measles and Mumps are both caused by viruses. Vaccines such as the MMR (Measles, Mumps, and Rubella) use live-attenuated viruses to provide immunity against these viral pathogens.

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Why other options are incorrect:

• • Cholera vaccine: Cholera is caused by the bacterium Vibrio cholerae, making this a bacterial vaccine.
• • Typhoid vaccine: Typhoid fever is caused by the bacterium Salmonella Typhi, which is a bacterial infection.
• • Anti-rickettsial vaccine: Rickettsia are a specific group of gram-negative bacteria (obligate intracellular parasites), not viruses.

Explanation: Autogenous vaccines are custom-made vaccines prepared from cultures of microorganisms (bacteria or viruses) isolated from a particular patient. They are then used to treat that same patient to target the specific strain causing their infection.

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Why other options are incorrect:

• • Anti viral vaccines: These are vaccines designed to protect against viruses in general, usually mass-produced for the general population.
• • Anti bacterial vaccines: These are vaccines designed to protect against bacterial infections, produced using standard lab strains.
• • None of these: This is incorrect because ‘Autogenous’ specifically describes vaccines derived from the patient’s own infecting microbe.

Explanation: Sub-cellular vaccines (also known as subunit or acellular vaccines) are prepared from specific components of a microbe, such as its toxins, proteins, or polysaccharides, rather than using the entire cell.

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Why other options are incorrect:

• • Cellular vaccines: These contain the whole microbe (either killed or live), not just isolated chemicals or toxins.
• • Attenuated vaccines: These consist of whole, live microorganisms that have been weakened so they cannot cause disease.
• • Heterologous vaccines: These use a pathogen from a different species that is closely related enough to provide cross-protection (e.g., using cowpox virus to protect against smallpox).

Explanation: Live-attenuated vaccines use a weakened (attenuated) form of the germ that causes a disease. Rubella (German measles) and BCG (Bacille Calmette-Guérin, used against Tuberculosis) are classic examples of vaccines containing live, weakened pathogens.

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Why other options are incorrect:

• • Polio & TAB: While Oral Polio Vaccine (OPV) is live, TAB (Typhoid-Paratyphoid A & B) is an inactivated (killed) bacterial vaccine.
• • Diphtheria & Tetanus: These are Toxoid vaccines, made from inactivated toxins produced by the bacteria, not live germs.
• • Hepatitis A & Rabies: These are typically Inactivated (killed) vaccines, where the virus has been destroyed so it cannot replicate.

Explanation: The DPT vaccine is a triple antigen combination vaccine designed to provide simultaneous immunity against three highly infectious bacterial diseases: Diphtheria, Pertussis (whooping cough), and Tetanus.

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Why other options are incorrect:

• • Option a: This is incomplete as it leaves out the Pertussis component (the ‘P’ in DPT).
• • Option b: This is incomplete as it leaves out the Tetanus component (the ‘T’ in DPT).
• • Option d: This is incorrect because option ‘c’ provides the full and accurate answer.

Explanation: The Rabies vaccine used for humans is an inactivated (killed) vaccine. It is not administered as a live attenuated vaccine to humans.

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Why other options are incorrect:

• • Influenza: Available in both inactivated (injection) and live attenuated (nasal spray/LAIV) forms.
• • Measles: This is a classic example of a live attenuated vaccine (usually part of the MMR vaccine).
• • Polio: Available as both an inactivated vaccine (IPV/Salk) and a live attenuated oral vaccine (OPV/Sabin).

Explanation: HIV can be transmitted through blood, semen, and vaginal fluids. These body fluids can contain sufficient amounts of the virus to infect another person when they come into contact with mucous membranes or enter the bloodstream.

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Why other options are incorrect:

• • Blood: HIV can be transmitted through infected blood, but this is not the only route.
• • Semen: HIV is present in semen and can be sexually transmitted, but this is not the only route.
• • Vaginal fluid: HIV can also be present in vaginal secretions, but this alone is not the complete answer.

Explanation: Animals possess natural species immunity against certain human-specific pathogens. Both Vibrio cholerae (causing cholera) and Salmonella typhi (formerly S. typhosa, causing typhoid fever) primarily infect humans, and most animals are naturally resistant to these infections.

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Why other options are incorrect:

• • V. Cholera: Animals are generally resistant, but this is only one of the correct organisms.
• • S. typhosa: Animals are also naturally resistant, but this alone is not the complete answer.
• • None of these: Incorrect because animals show natural immunity to both of these human-specific pathogens.

Explanation: Artificial active immunity is induced by vaccines. Inoculating the body with attenuated (weakened) organisms triggers the body’s own immune system to produce antibodies and memory cells without causing the full-blown disease.

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Why other options are incorrect:

• • Passive immunity: This involves receiving pre-formed antibodies (like via the placenta or antivenom) rather than the body producing them.
• • Natural active immunity: This is acquired through natural clinical or subclinical infection rather than intentional inoculation.
• • Local immunity: This refers to immunity at a specific site (like the gut or nose), usually mediated by IgA, rather than the systematic artificial process.

Explanation: Attenuation is the process of reducing the virulence of a pathogen while retaining its ability to stimulate immunity. This can be achieved by growing the organism at non-optimal temperatures, passing it through less susceptible animal species, or continuously cultivating it in the presence of antagonistic substances. Therefore, any one of the above methods can be used for attenuation.

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Why other options are incorrect:

• • Growing at a higher temperature: A valid method of attenuation, but not the only one.
• • Passage through less susceptible animals: Also a recognized attenuation technique, but not the sole method.
• • Continuous cultivation with antagonistic substances: Can reduce virulence, but is only one approach.
• • None of these: Incorrect because all the listed methods can be used for attenuation.

Explanation: Passive immunity is temporary because it involves the transfer of pre-formed antibodies from another source (like maternal antibodies or immunoglobulin injections). Since the recipient’s immune system is not actively producing these antibodies and no memory cells are created, the protection lasts only as long as the antibodies remain in circulation, typically 2 to 3 months.

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Why other options are incorrect:

• • 10 days: This is generally too short; IgG antibodies have a half-life of about 21–25 days, allowing protection to last longer than 10 days.
• • 10 years: This long-term protection is characteristic of Active Immunity, where memory cells are produced (e.g., after a Tetanus booster).
• • None of the above: Incorrect because option ‘b’ represents the scientifically accepted duration for the effectiveness of transferred antibodies.

Explanation: Various indicators can help in detecting autoimmunity, including hypergammaglobulinaemia, the presence of circulating autoantibodies, a favorable response to cortisone therapy, and lymphoid hyperplasia. Therefore, the correct answer is All of these.

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Why other options are incorrect:

• • Hyper gamma globulinaemia: A marker of increased antibody production, but not the only indicator.
• • Circulating antibodies: Autoantibodies are important markers, but additional findings are also considered.
• • Response to cortisone: Many autoimmune diseases improve with corticosteroid therapy, but this alone is not diagnostic.
• • Lymphoid hyperplasia: Commonly associated with autoimmune activity, but it is only one marker.

Explanation: The H antigen (flagellar antigen) is found on the flagella of bacteria. Flagella are the organs of locomotion, meaning they are only present in motile organisms. In serology (such as the Widal test for Typhoid), H antigens are used to identify specific strains of motile bacteria.

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Why other options are incorrect:

• • Non-motile organ: Non-motile bacteria lack flagella and therefore do not possess H antigens. They usually only possess O (somatic) antigens.
• • Both: This is incorrect because the definition of an H antigen is strictly tied to the presence of flagella (the motile apparatus).

Explanation: Antitoxins contain preformed antibodies that neutralize specific toxins. Since the antibodies are provided directly to the recipient rather than being produced by the recipient’s immune system, antitoxins confer passive immunization and provide immediate but temporary protection.

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Why other options are incorrect:

• • Active: Active immunization stimulates the body’s own immune system to produce antibodies and memory cells.
• • Both Active and Passive: Antitoxins provide only passive immunity because antibodies are supplied externally.
• • None of these: Incorrect because antitoxins are a classic example of passive immunization.

Explanation: Agglutination tests are widely used in microbiology for the identification of bacteria, typing of bacterial species, and the study of bacterial antigenic structures. Therefore, all the listed applications are correct.

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Why other options are incorrect:

• • Identification of isolated bacteria: A valid use of agglutination tests, but not the only one.
• • Typing of bacterial species: Agglutination is commonly used for serotyping, but this is only one application.
• • Study of antigenic structure of bacteria: Agglutination reactions help analyze bacterial antigens, but this is not the sole use.

Explanation: Agglutination tests are widely used in microbiology for the identification of bacteria, typing of bacterial species, and the study of bacterial antigenic structures. Therefore, all the listed applications are correct.

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Why other options are incorrect:

• • Identification of isolated bacteria: A valid use of agglutination tests, but not the only one.
• • Typing of bacterial species: Agglutination is commonly used for serotyping, but this is only one application.
• • Study of antigenic structure of bacteria: Agglutination reactions help analyze bacterial antigens, but this is not the sole use.

Explanation: For a substance to act as an anaphylactic antigen (allergen), it is generally protein in nature, possesses a relatively large molecular weight, and is soluble in tissue fluids. These properties enable it to induce IgE antibody production and trigger anaphylactic reactions upon subsequent exposure. Therefore, the correct answer is All of the above.

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Why other options are incorrect:

• • Protein in nature: A characteristic of many allergens, but not the complete answer.
• • Should have a large molecular weight: Important for antigenicity, but only one requirement.
• • Soluble in tissue fluids: Facilitates interaction with the immune system, but alone is insufficient.

Explanation: Asthma, allergic rhinitis, and urticaria are allergic (hypersensitivity) conditions characterized by local vasodilation, increased capillary permeability and secretion, and infiltration of eosinophils in tissues and blood. These changes are responsible for the typical symptoms of allergy and inflammation.

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Why other options are incorrect:

• • Local vasodilation: It is one feature of allergic reactions but not the complete pathology.
• • Increased capillary secretion: This contributes to swelling and mucus production but is only one aspect.
• • Excess eosinophils in tissue secretion and blood: Eosinophilia is characteristic of allergic diseases but does not represent the entire pathological process.
• • All of these: Correct answer because all the above features are involved in the pathology of these allergic conditions.

Explanation: The Schick test is a skin test used to determine whether or not a person is susceptible to diphtheria. It involves injecting a small amount of diluted diphtheria toxin intradermally; if a person lacks immunity, a local inflammatory reaction occurs.

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Why other options are incorrect:

• • Dick test: This test is used to determine susceptibility to Scarlet Fever (caused by Streptococcus pyogenes).
• • V-P (Voges-Proskauer) test: This is a biochemical test used in laboratories to detect acetoin production by certain bacteria to identify species.
• • Precipitin test: A general serological test used to identify the reaction between a soluble antigen and its corresponding antibody.

Explanation: Vaccines against plague and tularemia have been developed using various approaches, including chemical fractions of the causative bacteria, heat-killed preparations, formalin-inactivated suspensions, and live attenuated (avirulent) bacterial strains. Therefore, all of the listed methods can be used in vaccine preparation.

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Why other options are incorrect:

• • Chemical fraction of the causative bacteria: Can be used, but it is not the only method.
• • Heat killed suspension of virulent bacteria: Has been used in vaccine production, but it is not the sole approach.
• • Formalin inactivated suspension of virulent bacteria: Also used for vaccine preparation, but not exclusively.
• • Avirulent live bacteria: Live attenuated strains can induce immunity, but they are only one of several vaccine types.
• • All of these: Correct answer because all listed methods have been utilized in preparing plague and tularemia vaccines.

Explanation: Natural Killer (NK) cells are a type of cytotoxic lymphocyte critical to the innate immune system. They play a major role in the host-rejection of tumors and virally infected cells by releasing small cytoplasmic granules of proteins called perforin and granzyme that cause apoptosis.

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Why other options are incorrect:

• • B/T-cell lineage: NK cells are lymphoid cells, but they are distinct from B-cells and T-cells. They do not possess TCR (T-cell receptors) or BCR (B-cell receptors).
• • Previous exposure: Unlike the adaptive immune system (T and B cells), NK cells do not require prior sensitization or “memory” of an antigen to act; they are part of the body’s first line of defense.