8.2 Immunoglobulins
Key Takeaways
- Each immunoglobulin monomer has two heavy and two light chains, a Fab region (antigen binding, variable domains and CDRs) and an Fc region (effector functions, complement fixation, placental transfer).
- IgM is a pentamer with up to 10 theoretical binding sites, does not cross the placenta, and is the dominant class of naturally occurring ABO antibodies and efficient complement activators.
- IgG is a monomer, the only class that crosses the placenta via FcRn, and causes HDFN; IgG1 and IgG3 fix complement and bind macrophage Fc receptors strongly, while IgG2 is weak and IgG4 is essentially inert.
- IgA exists as a serum monomer and a secretory dimer; severe IgA deficiency with anti-IgA risks anaphylactic transfusion reactions and requires IgA-deficient or extensively washed components.
- IgG's small size cannot bridge red cell zeta potential, which is why IgG antibodies require antiglobulin testing while IgM agglutinates directly in saline.
Why Immunoglobulin Class Determines Clinical Behavior
Every antibody the blood bank detects belongs to one of five immunoglobulin classes, and the class -- not just the specificity -- determines whether that antibody agglutinates directly in saline, crosses the placenta, fixes complement, or requires the antiglobulin test to reveal itself. The SBB exam expects the ability to predict clinical behavior (HDFN severity, transfusion reaction type, in vitro reactivity pattern) directly from immunoglobulin structure and class, rather than memorizing isolated facts.
Basic Immunoglobulin Structure
Every immunoglobulin monomer is built from two identical heavy chains and two identical light chains, held together by disulfide bonds into a Y-shaped molecule (or, for IgM, a star-shaped pentamer). Light chains occur as one of two types, kappa or lambda -- a single molecule uses only one type, never both. Each chain contributes a variable (V) region, containing the hypervariable complementarity-determining regions (CDRs) that form the actual antigen-binding surface, and a constant (C) region.
Structurally, the molecule divides into two functional fragments. The Fab (fragment, antigen-binding) region contains the variable domains and is responsible for antigen recognition -- the "business end" that determines specificity. The Fc (fragment, crystallizable) region is built entirely from heavy-chain constant domains and mediates effector functions: complement fixation, binding to Fc receptors on macrophages and other immune cells, and, for IgG, placental transfer via the neonatal Fc receptor. A flexible hinge region between Fab and Fc lets the two antigen-binding arms swing and adjust the angle between them, a property that matters for how efficiently the molecule bridges two antigen sites -- directly relevant to agglutination, covered in the next section.
Classes and Subclasses
The heavy-chain constant region determines immunoglobulin class (isotype): mu (IgM), gamma (IgG), alpha (IgA), delta (IgD), or epsilon (IgE).
IgM circulates predominantly as a pentamer -- five monomeric units joined by a J (joining) chain -- giving it a molecular weight of roughly 900,000 Da, ten theoretical antigen-binding sites (though steric constraints typically limit functional valence to about five), and the largest physical size of any immunoglobulin. Its size and multivalency make it the most efficient complement activator and give it high avidity. IgM does not cross the placenta. It is the class of most naturally occurring antibodies (ABO, many cold agglutinins, some Lewis and P1 antibodies) and typically reacts best at room temperature or colder, agglutinating saline-suspended red cells directly without antiglobulin reagent.
IgG is a monomer (MW ~150,000 Da, two antigen-binding sites) and is the only class that crosses the placenta, via active transport through the neonatal Fc receptor (FcRn) in the syncytiotrophoblast -- the class responsible for hemolytic disease of the fetus and newborn. IgG has four subclasses that behave very differently: IgG1 and IgG3 fix complement efficiently and bind Fc receptors on macrophages strongly, making them the subclasses most associated with severe HDFN and extravascular hemolysis; IgG2 activates complement weakly; IgG4 essentially does not fix complement and binds Fc receptors poorly. IgG typically reacts optimally at 37 degrees C ("warm" antibodies) and, because of its small size, generally cannot bridge the distance between red cells in saline -- it requires antihuman globulin (the indirect antiglobulin test) to produce visible agglutination.
IgA exists as a monomer in serum but as a dimer in secretions, linked by a J chain and stabilized by a secretory component derived from the polymeric immunoglobulin receptor on mucosal epithelium -- the dominant antibody of mucosal immunity (tears, saliva, colostrum, respiratory and GI secretions). IgA carries specific transfusion-medicine significance: patients with severe IgA deficiency who have developed anti-IgA are at risk of severe or fatal anaphylactic transfusion reactions on exposure to IgA in donor plasma, and require IgA-deficient components or extensively washed cellular products.
IgD is a monomer expressed mainly as a B-cell surface receptor alongside IgM on naive B cells; it has minimal independent serum function and no significant blood bank relevance. IgE is present at the lowest serum concentration of any class and binds high-affinity Fc receptors on mast cells and basophils, mediating immediate hypersensitivity and allergic transfusion reactions.
| Class | Structure | MW (approx.) | Complement Fixation | Crosses Placenta | Blood Bank Relevance |
|---|---|---|---|---|---|
| IgM | Pentamer | ~900,000 | Most efficient | No | ABO, cold agglutinins; direct saline agglutination |
| IgG | Monomer | ~150,000 | IgG1, IgG3 strong | Yes | HDFN, extravascular hemolysis, IAT-detected |
| IgA | Monomer/dimer | ~160,000-385,000 | Poor | No | Anaphylactic reaction in IgA-deficient recipients |
| IgD | Monomer | ~180,000 | None | No | Minimal |
| IgE | Monomer | ~190,000 | None | No | Allergic transfusion reactions |
Physical Properties and Practical Implications
Physical differences map directly onto laboratory behavior. IgM's larger sedimentation coefficient (19S versus IgG's 7S) and greater molecular size are why it sediments and agglutinates more readily without enhancement. IgG's smaller size and lower valence (2 versus IgM's roughly 5-10 functional sites) explain why IgG-only antibodies frequently show no reaction at immediate spin but become reactive only after 37 degree C incubation and antiglobulin addition -- a pattern the SBB exam uses constantly to test whether a candidate can infer immunoglobulin class purely from a described reaction phase.
Which immunoglobulin class circulates as a pentamer, does not cross the placenta, and is the class of most naturally occurring ABO antibodies?
Which IgG subclasses fix complement most efficiently and bind Fc receptors on macrophages most strongly, making them the subclasses most associated with severe extravascular hemolysis and HDFN?
A patient with severe IgA deficiency has developed anti-IgA and requires transfusion. What is the primary risk from a standard plasma-containing component, and how is it prevented?
Which immunoglobulin fragment contains the variable-domain complementarity-determining regions responsible for antigen specificity, as distinct from the fragment that mediates complement fixation and placental transfer?