IgG antibodies are warm-reactive antibodies, meaning they react best at body temperature (37°C).

These antibodies are clinically significant in blood banking because they can cross the placenta and cause hemolytic transfusion reactions or hemolytic disease of the newborn (HDN).

Why others are incorrect:

• a) IgM:
  Reacts best at cold temperatures (20–24°C) — detected in the immediate-spin phase.

• c) IgA:
  Found mainly in secretions (tears, saliva, mucous) — not typically involved in RBC antigen reactions.

• d) IgE:
  Involved in allergic (hypersensitivity) reactions, not blood group antibody reactions.

An alloantibody is an antibody produced in response to antigens from another individual of the same species that are not present in the antibody producer.

These antigens are called alloantigens, and they differ between individuals due to genetic variation — such as blood group antigens (ABO, Rh, etc.).

Why others are incorrect:

• a) Antibody produced against self-antigens:
  That’s an autoantibody (seen in autoimmune diseases).

• c) Antibody produced by the complement pathway:
  Complement does not produce antibodies; it’s a protein system that assists antibody action.

• d) Antibody formed without antigen exposure:
  Some “naturally occurring” antibodies (like anti-A, anti-B) may seem spontaneous but still arise from environmental antigen exposure — not truly “without exposure.”

IgM is the largest immunoglobulin and the only one with a pentamer structure in its secreted form.
This means it is made up of five basic immunoglobulin (Y-shaped) units linked together by a J (joining) chain.

The immunoglobulin with a pentamer structure is IgM.

• IgA can be monomer (in serum) or dimer (in secretions).

• IgG is monomeric.

• IgM is pentameric (with a J chain).

• IgE is monomeric.

Antigen–antibody reactions in serologic tests are affected by the physical and chemical conditions of the test environment:

• a) pH, temperature, and ionic strength → Yes. These are well-known factors that influence the binding and agglutination in immunohematology.

• b) Blood pressure and glucose level → No, these are physiological factors, not direct factors in in vitro antigen–antibody reactions.

• c) Platelet count → Unrelated to antigen–antibody binding in serology.

• d) Hemoglobin type → Not a direct factor in antigen–antibody reaction conditions.

The indirect antiglobulin test (IAT) is used to detect unexpected antibodies in the patient’s serum by incubating serum with reagent red cells and adding AHG to visualize IgG antibody binding.

• a) Reverse ABO testing – usually immediate spin, not IAT.

• b) Immediate spin crossmatch – detects ABO incompatibility, not IAT.

• c) C antigen testing – direct agglutination, not IAT.

Here’s a quick breakdown to clarify the terms:

• d) Affinity: This is the correct term. It refers to the strength of a single binding site interaction between one antibody and one antigen.

• a) Avidity: This is related but different. It refers to the overall strength of binding between an antibody (which can have multiple binding sites) and a multivalent antigen (which has multiple epitopes). Avidity is the combined strength of all the individual affinity interactions.

• b) Specificity: This refers to the ability of an antibody to recognize and bind to a unique antigen, not the strength of that binding.

• c) Titer: This is a measurement of the concentration or amount of an antibody in a sample, typically determined by the highest dilution that still gives a positive reaction.

IgM is a pentameric antibody with multiple binding sites, making it highly effective at agglutinating (clumping) red blood cells at room temperature. Its large size and high valence allow it to efficiently cross-link antigens on different cells.

Some transfusion-transmissible infections do not have routine laboratory screening tests in all blood banks, especially in areas where the infection is endemic. For these infections, donor history and questioning are the primary strategy to prevent transmission.

• a) Trypanosoma cruzi – Blood donations in the US are screened with a laboratory test (antibody test), so not solely by questioning.

• c) HCV (Hepatitis C virus) – Routinely screened by lab tests (NAT, antibody), so not solely questioning.

• d) CMV (Cytomegalovirus) – Not routinely screened by lab tests for all donations, but CMV-safe blood is often provided by selecting CMV-negative donors (which requires lab testing) or leukoreduction. It’s not solely dependent on questioning.

• This is the standard, required temperature range for storing and transporting apheresis platelets. Storing them at this temperature is essential to maintain their viability and function. If platelets are refrigerated, they undergo changes that quickly render them non-viable and are rapidly cleared from the recipient’s circulation upon transfusion.

Why the other options are incorrect:

• a) 1-6°C: This is the standard transport and storage temperature for Red Blood Cells and other components that require refrigeration.

• b) 1-10°C: This range is too cold for platelets and too warm for stable refrigeration of red blood cells. It is not a standard transport temperature for any major blood component.

• c) 18-20°C: While close, this range’s lower limit of 18°C is slightly below the required standard of 20°C and is not the specified range in guidelines.

The zone of equivalence is the optimal proportion of antigen and antibody in a precipitation or agglutination reaction where both are in roughly equal amounts, leading to maximum lattice formation and visible reaction.

• a) Excess antigen present → That’s the postzone effect (inhibition).

• b) Equal antigen and antibody concentration → Essentially correct for the equivalence zone.

• c) Excess antibody present → That’s the prozone effect (inhibition).

• d) Absence of antigen → No reaction.

Before donating blood, every donor must meet certain minimum hemoglobin (Hb) requirements to ensure donor safety and maintain blood quality.

For male donors, the minimum acceptable hemoglobin level is 13.0 g/dL.

Why others are incorrect:

• a) 12.0 g/dL: Too low — below safe limits for either gender.

• b) 12.5 g/dL: Applies to female donors, not males.

• d) 13.5 g/dL: Higher than the required minimum; acceptable but not the cutoff.

• Most blood group systems (e.g., ABO, MN, Kell, Duffy) are located on autosomes and exhibit codominant inheritance.

• Codominance means that if an individual inherits different alleles from each parent, both traits are expressed (e.g., blood type AB expresses both A and B antigens).

• They are not sex-linked, and while some traits can be recessive, the predominant pattern for blood groups is codominant.

• 2,3-Diphosphoglycerate (2,3-DPG) in RBCs binds to hemoglobin and facilitates oxygen release to tissues.

• During storage of RBCs, 2,3-DPG levels gradually decrease, reducing the oxygen-releasing capacity of hemoglobin.

• Platelets, FFP, and Cryoprecipitated AHF do not contain significant hemoglobin, so 2,3-DPG is not relevant for them.

This is why stored blood may have slightly reduced oxygen delivery, which usually normalizes after transfusion as 2,3-DPG levels recover in the recipient.

In genetics, a haplotype is defined as a group of genes, or more specifically, alleles (different versions of the same gene), that are located on a single chromosome and are inherited together from one parent.

Why the other options are incorrect:

• a) Allele: An allele is a specific version of a single gene. For example, *HLA-A*02:01* and *HLA-A*03:01* are different alleles of the HLA-A gene. The question refers to the linked set of multiple HLA genes, not a single variant.

• b) Trait: A trait is the observable physical or functional expression of a gene or set of genes (e.g., having red hair or blood type A). The linked HLA genes themselves are not a trait; they are the genetic instructions that lead to the expression of HLA antigens, which is the phenotype.

• c) Phenotype: The phenotype is the observable characteristic resulting from the genotype.

The memory (secondary) immune response occurs when the body is exposed again to the same antigen after a previous (primary) exposure.
During the first exposure, memory B and T cells are created and remain in the body for future protection.

Why others are incorrect:

• a) Lower antibody levels: 
  Memory response produces higher antibody levels.

• b) Slower antibody production: 
  It’s actually much faster than the primary response.

• d) Absence of antigen recognition: 
  The memory response depends on rapid antigen recognition by memory cells.

The complement system is a group of plasma proteins that enhance (or “complement”) the effects of the immune response — particularly antibody-mediated reactions.

When the complement system is activated (especially via the classical pathway triggered by antigen–antibody complexes), it can result in several biological effects — one of the most important being cell lysis.

Why others are incorrect:

• b) Decreased antibody production:
  Complement does not regulate antibody synthesis; that’s controlled by B cells.

• c) Loss of plasma volume:
  Not a direct result of complement activation.

• d) Increase in hemoglobin synthesis:
  Unrelated to complement; hemoglobin production occurs in developing red cells in the bone marrow.

• d) Povidone iodine: This is the most common and widely recommended antiseptic for preparing the venipuncture site prior to blood donation. It is highly effective at reducing skin bacteria and preventing contamination of the blood collection bag.

Why the other options are incorrect:

• a) Hypochlorite: This is a bleach solution. It is too harsh for use on skin and is not used for venipuncture site preparation.

• b) Green soap: This is a detergent used for general cleaning of skin, often before a surgical scrub, but it is not an adequate antiseptic by itself for blood donation venipuncture.

• c) 10% acetone: Acetone is a solvent and is not used as a skin antiseptic for this purpose. It can be irritating and damage the skin.

Agglutination in immunology and blood banking refers to the process where particulate antigens (such as red blood cells or bacteria) clump together when mixed with corresponding antibodies.

• a) Destruction of red cell membranes → That’s hemolysis, not agglutination.

• b) Binding of antigen and antibody causing visible clumping → Yes, that’s the definition of agglutination.

• c) Separation of serum proteins → No, that’s electrophoresis or centrifugation.

• d) Denaturation of plasma enzymes → No, that’s unrelated.

• MAC (Membrane Attack Complex) → final step of complement activation, which forms a pore in target cell membranes, leading to lysis.

• Components of MAC: C5b, C6, C7, C8, and multiple C9 molecules.

• a) C1q, C1r, C1s → part of classical pathway initiation, not MAC.

• b) C3a, C3b, C3d → involved in opsonization and inflammation, not MAC.

• c) C5a, C5b → C5b initiates MAC, but not the complete MAC.

• d) C5b through C9 → Correct, these form the pore structure of MAC.

In immunohematology, sensitization means that an antibody has bound to its specific antigen on the red cell, but this binding has not caused visible agglutination.

• Example: IgG antibodies often sensitize RBCs, but to see agglutination, an antiglobulin test (Coombs test) is needed.

Now the options:

• a) Visible red cell agglutination → No, that’s agglutination, not sensitization.

• b) Binding of antibody to antigen without visible agglutination → Yes, that’s the definition.

• c) Cell lysis by complement activation → No, that’s hemolysis.

• d) Denaturation of antibodies → Unrelated.

The antiglobulin reagent (Coombs reagent) contains antibodies directed against human immunoglobulins (like IgG) and/or complement components (like C3d).

• a) Free serum proteins → No, the reagent reacts with antibodies or complement bound to red cells, not free in serum.

• b) Antibody-coated red blood cells → Yes — the reagent bridges between antibody molecules on different RBCs, causing agglutination if they are coated with IgG or complement.

• c) Plasma enzymes → Unrelated.

• d) Hemoglobin variants → Detected by electrophoresis, not antiglobulin test.

Let’s carefully analyze this question. It’s about antibodies in immunohematology and their clinical significance.

• Anti-P – Can be clinically significant; can cause hemolytic transfusion reactions and HDFN in rare cases.

• Anti-P1 – Usually clinically insignificant; generally cold-reacting IgM antibody and rarely causes hemolysis.

• Anti-Pk – Rare, but can be significant if reactive at body temperature.

• Anti-p – Rare, usually clinically significant; associated with hemolytic disease of the newborn and hemolytic transfusion reactions.

Innate immunity (also called natural or nonspecific immunity) is the first line of defense against infection — it is present from birth and functions without prior exposure to any specific antigen.

It provides immediate protection against a wide range of pathogens.

Why others are incorrect:

• a) Acquired immunity:
  Develops after exposure to an antigen (through infection or vaccination).

• b) Adaptive immunity:
  Another term for acquired immunity — requires prior sensitization.

• d) Delayed response:
  Refers to a type IV hypersensitivity reaction, not an overall immune category.

An antigen that is located on the surface of red blood cells (RBCs) is called a red cell antigen (or erythrocyte antigen).

These antigens are specific molecular structures — usually proteins, carbohydrates, or glycoproteins — embedded in or attached to the RBC membrane.

Why others are incorrect:

• a) Plasma antigen: Not a standard term; plasma contains soluble proteins, not cell-bound antigens.

• c) Serum antigen: Serum contains antibodies, not RBC antigens.

• d) Complement antigen: Complement is a group of proteins, not antigens.

Hemagglutination in blood banking refers to the clumping of red blood cells due to antibody binding to red cell antigens.

• a) Detect viruses → Viral hemagglutination is a different context (e.g., influenza), not the primary use in blood banking.

• b) Identify red cell antigens and antibodies → Yes — blood grouping, antibody screening, and crossmatching rely on hemagglutination.

• c) Determine plasma glucose → Unrelated; uses chemistry methods.

• d) Analyze clotting factors → Uses coagulation tests, not hemagglutination.

IgG is the only class of antibody that can cross the placenta from the mother to the fetus. If a mother has IgG antibodies against the fetus’s red blood cell antigens (e.g., anti-D in Rh disease), it can cause hemolytic disease of the newborn.

Testing donor blood for syphilis (using a serologic test) is a required screening test in most countries, including per FDA regulations in the US.

• a Complete Rh phenotyping: Not required; only D antigen is routinely tested.

• b Anti-CMV testing: Not required for all donors; only for specific at-risk recipients.

• c Direct antiglobulin test: Not a routine donor screening test.

• 2,3-DPG (2,3-diphosphoglycerate) is an intermediate in glycolysis within RBCs.

• Its main role is to bind hemoglobin and reduce its affinity for oxygen, facilitating oxygen release to tissues.

• a) Protect RBCs from HMP shunt degradation → Not true.

• b) Generate ATP anaerobically → Glycolysis does generate ATP, but that’s not the role of 2,3-DPG itself.

• c) Maintain hemoglobin Fe²⁺ → That’s the role of methemoglobin reductase, not 2,3-DPG.

• d) Increase the release of O₂ from oxyhemoglobin

• c) Positive test for Trypanosoma cruzi: This is the causative agent of Chagas disease. A positive test indicates a chronic, transmissible infection that can be passed to a transfusion recipient. This is a cause for indefinite deferral to protect the blood supply.

Why the other options are incorrect:

• a) Reactive test for Babesia species: While this is a cause for deferral, it is often not indefinite. Donors may be eligible again after a certain period or after successful treatment, depending on the blood bank’s specific protocols.

• b) Residence in an endemic malaria region for 5 years: This would lead to a temporary deferral (typically 1 to 3 years after leaving the area), not an indefinite one.

• d) History of chicken pox vaccination: This is not a cause for deferral. Donors are eligible as long as they are feeling healthy and well on the day of donation.

Cryoprecipitate is rich in fibrinogen, factor VIII, von Willebrand factor, and factor XIII.
It is the most targeted product to correct hypofibrinogenemia in conditions like DIC when fibrinogen levels are critically low.

• Fresh Frozen Plasma (b) contains all coagulation factors but has lower fibrinogen concentration per unit volume.

• Whole Blood (a) and Platelets (d) do not provide sufficient fibrinogen.

IgG is the only immunoglobulin class that can cross the placenta from mother to fetus, providing passive immunity to the newborn.

• IgM: too large.

• IgA: found in secretions, not transferred.

• IgE: involved in allergies, not transferred.

Let’s go through the options.

• a) LISS (Low Ionic Strength Solution) → Works by reducing ionic strength, which increases the rate of antibody uptake by decreasing the electrostatic repulsion (zeta potential) between RBCs and antibodies.

• b) Saline → Normal ionic strength, does not enhance uptake; used for washing and suspension.

• c) Albumin → Can enhance agglutination by reducing zeta potential and promoting antibody bridging, but LISS is more directly described as enhancing antibody uptake by this mechanism.

• d) Enzyme solution → Works by removing sialic acid residues (reducing negative charge, thus zeta potential), but the question specifies enhances antibody uptake — LISS is designed for that in incubation phase, while enzymes alter the antigenic structure.

• Enhancement media (like LISS, PEG, albumin) are used in blood banking to promote agglutination.

• They work by reducing the repulsive forces between red cells, which allows antibodies to bridge antigens more easily.

• The repulsive force between negatively charged red cells is called the zeta potential.

• Hydrophilic forces → not the main factor here.

• Low ionic potential → LISS reduces ionic strength, but the effect is ultimately to lower zeta potential.

• Van der Waals forces → weak attraction, not the main mechanism.

B cells (B lymphocytes) are responsible for producing antibodies, including blood group antibodies, when they differentiate into plasma cells.

• T cells (b) are involved in cell-mediated immunity, not antibody production.

• NK cells (c) are part of innate immunity and kill infected or cancerous cells.

• Dendritic cells (d) are antigen-presenting cells that help activate T cells.

Let’s go step by step.

• Primary immune response → the first time the immune system encounters an antigen.

• The body first produces IgM, which is a pentamer and very effective in initial defense.

• IgG comes later, after class switching.

• IgA is mainly in mucosal secretions, not the first systemic response.

• IgE is involved in allergic responses and parasites, also not first.

• Hemophilia B is caused by a deficiency of Factor IX.

• Treatment involves replacing Factor IX using Factor IX concentrate (either plasma-derived or recombinant).

• Factor VIII (b) is for hemophilia A.

• Cryoprecipitate (c) contains Factor VIII and fibrinogen, not Factor IX.

• DDAVP (d) is used in mild hemophilia A or von Willebrand disease.

Check cells (Coombs control cells) are IgG-sensitized red cells.
If the AHG test was negative, adding check cells should cause agglutination, confirming that:

• The AHG reagent was present and active

• The washing step was adequate (no free IgG neutralized the AHG)
  This is a quality control step to validate negative AHG results.

The DAT (direct Coombs test) involves testing red blood cells taken directly from a patient to see if they are already coated with antibody or complement in vivo.

• a) Free serum antibodies → No, that’s the indirect antiglobulin test (IAT).

• b) Antibodies or complement bound to red cells in vivo → Yes, exactly. DAT detects in vivo sensitization.

• c) Hemoglobin concentration → No, unrelated.

• d) Plasma proteins → No, not specific.

Enhancement media like LISS (Low Ionic Strength Salt solution) and PEG (Polyethylene Glycol) are used in blood bank serology to speed up and increase the sensitivity of antigen–antibody reactions.

• a) Increase antibody production → No, they don’t affect the body’s production of antibodies; they work in in vitro tests.

• b) Accelerate antigen–antibody binding → Yes — LISS reduces the ionic strength, which increases the rate of antibody binding to RBC antigens; PEG concentrates antibodies by excluding water, enhancing reactivity.

• c) Reduce hemolysis → Not their primary role; hemolysis is unrelated here.

• d) Dilute serum proteins → The opposite — PEG concentrates, LISS does not dilute significantly in the intended use.

The hh genotype means the person has the Bombay phenotype. Even though they have the AB gene for the ABO system, they cannot produce the H antigen, which is the precursor needed to form A or B antigens.
Without H antigen, neither A nor B antigen can be expressed on the red cells.
Thus, with anti-A and anti-B, no agglutination occurs → type as O.

The Immediate-spin (IS) phase is the cold phase of antibody testing, performed at room temperature (20–24°C) or below.

This phase primarily detects IgM antibodies, which are:

• Large pentameric molecules

• Efficient at agglutination

• React best at lower temperatures (cold-reactive)

Why others are incorrect:

• a) 37°C incubation phase:
  Detects IgG antibodies, which react optimally at body temperature.

• c) Antiglobulin phase (AHG test):
  Enhances detection of IgG antibodies that do not cause direct agglutination.

• d) Enzyme phase:
  Enhances IgG antibody reactivity by exposing antigen sites on red cells.

An antigen is any substance—such as a protein, polysaccharide, or toxin—that the immune system recognizes as foreign, which can then trigger the production of antibodies or activate immune cells.

The complement system is a group of plasma proteins that work with antibodies to destroy pathogens. When activated, complement components can lyse (break down) cells, enhance phagocytosis, and promote inflammation, thereby strengthening the immune response.

The immune system is made up of cells that defend the body against infection and disease.
These include white blood cells (leukocytes) such as:

• Lymphocytes → B cells, T cells, and NK cells (specific/adaptive immunity)

• Monocytes → Become macrophages; perform phagocytosis (innate immunity)

• Basophils → Release histamine in allergic and inflammatory responses (innate immunity)

• Solid phase adherence (SPA) assays are used to detect antibodies bound to antigens in wells coated with red cell antigens or patient antibodies.

• How it works:

  • If antibody is present, indicator red cells will bind to the well bottom → agglutination.

  • If antibody is absent (negative result), indicator red cells cannot bind → they settle freely at the bottom, forming a pellet.

• a) A red blood cell pellet in the bottom of the well → ✅ Correct for negative result

• b) Diffuse pattern → usually seen in positive adherence (agglutination)

• c) Red cell clumps symmetrically → positive reaction

• d) Red supernatant / lysis → not applicable to SPA assays

• When there is too much antibody compared to antigen, the antibody binds all available antigen sites but cannot form the lattice needed for visible agglutination. This is called the prozone effect. ✅

• Postzone effect → occurs when antigen is in excess, also preventing visible agglutination.

• Dosage effect → refers to weaker reactions with heterozygous cells due to less antigen expression.

• pH effect → changes in reaction due to altered pH, not antibody excess.

• Direct hemagglutination requires an antibody that can bind multiple red cells simultaneously.

• IgM is a pentamer, so it has 10 antigen-binding sites, making it very efficient at agglutination, even at room temperature.

• IgG is a monomer (2 binding sites), usually too small for direct agglutination; it often requires indirect methods (like the Coombs test).

• IgA is mostly in secretions, not efficient at direct RBC agglutination.

• IgE is monomeric and involved in allergy, not hemagglutination.

In blood banking (immunohematology), strict laboratory safety measures are essential to prevent exposure to bloodborne pathogens such as HIV, HBV, and HCV, and to ensure the accuracy of results.

Among all safety practices, the most critical is personal protection — particularly wearing gloves when handling blood or blood products.

Why others are incorrect:

• b) Using only glass pipettes:
  Glass pipettes can break easily, increasing risk of injury and contamination — most labs now use plastic, disposable pipettes.

• c) Avoiding refrigeration of samples:
  Incorrect — blood samples should be refrigerated when not tested immediately to preserve cell and serum integrity.

• d) Mixing patient and donor samples directly:
  Extremely unsafe — can cause cross-contamination and false results; testing is always performed in separate, controlled reactions.

The specific immune response relies on the recognition of antigens by specific antibodies and immune cells (like B cells and T cells). This antigen–antibody interaction is the central event that triggers a targeted, adaptive immune response.

• The window period is the early stage of an infection when the donor has been infected but the infection is not yet detectable by standard screening tests.

• During this period, blood may appear safe but can still transmit pathogens to the recipient.

Other options:

• a) Pathogen reduction technology failure – Rare; not the primary cause.

• c) Leukocyte-reduction failure – Reduces certain reactions, but doesn’t prevent viral transmission.

• d) Donor history questionnaire not completed – Important, but less critical than the window period risk.

All donor blood must be tested for mandatory infectious agents to ensure recipient safety.
According to AABB and most national standards, testing for syphilis using serological methods (e.g., RPR, VDRL, or treponemal tests) is required for every donation.

Why the others are incorrect:

• a) Complete Rh phenotyping Only required in special situations (e.g., for patients with multiple transfusions or rare phenotypes), not all donors
• .b) Anti-CMV testing Optional; leukoreduction or special CMV-negative units are used for high-risk recipients, not mandatory for all donors.
• c) Direct antiglobulin test (DAT) Used to detect antibody-coated RBCs in patients, not routine donor screening.

• Fresh Frozen Plasma (FFP) must be frozen at –20°C or lower within 8 hours of collection to preserve clotting factors.

• Storage at –40°C is also acceptable and used for long-term storage, but standard FFP is usually kept at –20°C.

• 4°C is for plasma after thawing (short-term use), and 37°C would destroy clotting factors.

The indirect antiglobulin test (IAT) is used in blood banking to detect antibodies in a patient’s serum that can react with known antigens on reagent red blood cells.

• a) Antigens on donor cells → No, antigens are directly detected by direct agglutination or other methods, not IAT.

• b) Antibodies present in patient serum → Yes: IAT involves incubating patient serum with reagent RBCs, then adding anti-human globulin to detect antibody binding.

• c) Complement proteins → IAT can detect complement fixed to RBCs if anti-C3 is in the antiglobulin reagent, but the primary purpose is detecting serum antibodies (IgG).

• d) Clotting factors → Unrelated to IAT.

An immunoglobulin (antibody) molecule has a basic Y-shaped structure, and its fundamental structural unit consists of:

• 2 identical heavy (H) chains

• 2 identical light (L) chains (either κ or λ type)

These chains are held together by disulfide bonds.

Why others are incorrect:

• a) One heavy and two light chains: → Incorrect ratio

• c) One heavy and one light chain: → Half molecule (Fab fragment), not complete antibody

• d) Four heavy chains only: → Missing light chains necessary for antigen binding

• ABO antibodies (mostly IgM) bind to A or B antigens on transfused red cells.

• IgM is very effective at activating complement.

• Complement activation occurs via the classical pathway, which is triggered by antigen-antibody complexes.

• a) Alternative → activated spontaneously or by pathogen surfaces, not antibody-dependent.

• b) Classical →  Correct, triggered by antibodies (IgM or IgG) bound to antigen.

• c) Lectin → triggered by mannose-binding lectin on pathogens, not relevant here.

• d) Polyclonal → not a complement pathway.

The primary immune response is the body’s first reaction to a new (initial) antigen exposure.

When an antigen enters the body for the first time:

1. Naïve B lymphocytes recognize the antigen.

2. They become activated and begin to produce antibodies, mainly IgM first.

3. Later, class switching occurs, leading to IgG production.

4. Memory cells are also formed for faster response upon re-exposure.

Why others are incorrect:

• a) Secondary immune response:
  Occurs after a second exposure to the same antigen — faster and stronger due to memory cells.

• b) Delayed hypersensitivity reaction:
  A T-cell–mediated reaction (Type IV hypersensitivity), not the initial immune response.

• d) Memory response:
  Another term for secondary immune response, not the first exposure.

Immunohematology (also known as Blood Banking) focuses on the study of antigen-antibody reactions as they relate to blood cells. Its primary application is in pre-transfusion testing (like blood typing and crossmatching), antibody identification, and the investigation of hemolytic disease of the fetus and newborn, all of which are critical for ensuring the safety of blood transfusions.

• Erythropoiesis → the process of red blood cell (RBC) production.

• Key cytokines:

  • Erythropoietin (EPO): Major hormone/cytokine stimulating RBC progenitor proliferation and differentiation.

  • Interleukin 3 (IL-3): Supports early hematopoietic progenitors, including erythroid lineage.

• GM-CSF → stimulates granulocyte and macrophage lineages, not specific for erythrocytes.

• Flt3 ligand → acts on hematopoietic stem cells, more general, not erythroid-specific.

The antiglobulin test, also known as the Coombs test, is a fundamental procedure in immunohematology (blood banking).
Its primary purpose is to detect red blood cells (RBCs) that have been coated with antibodies and/or complement components.

Why others are incorrect:

• a) Measure serum protein levels:
  Done using protein assays (e.g., total protein, albumin tests), not the Coombs test.

• c) Assess leukocyte function:
  White cell function is evaluated by immunologic or hematologic assays, not AHG testing.

• d) Evaluate coagulation status:
  Coagulation is measured by tests like PT, aPTT, and INR, not antiglobulin tests.

• AHG (anti-human globulin) reagents are used in Coombs tests to detect antibodies or complement on red cells.

• Polyspecific AHG → contains more than one specificity.

• It is designed to detect:

  • IgG antibodies bound to red cells

  • Complement components, mainly C3d

• Anti-IgM → not included because IgM usually causes direct agglutination at room temperature.