8.1 Immune Response

Why Immunology Underlies Every Transfusion Decision

The blood bank sits at the intersection of immunology and hematology: nearly every serologic reaction on the bench -- an unexpected antibody screen, a positive DAT, a transfusion-reaction workup -- is the immune system doing exactly what it evolved to do, directed at a red cell antigen instead of a pathogen. The SBB exam expects specialist-level fluency in the mechanisms behind those reactions, not only the pattern recognition tested at the technologist level. This section covers the primary and secondary immune response, the cellular players (B cells, T cells, macrophages), and the genetic basis of antibody diversity and antigen presentation.

Primary Versus Secondary (Anamnestic) Response

The primary immune response occurs on first exposure to an antigen. There is a lag phase of roughly 5-10 days before detectable antibody appears, because naive B cells must first be activated, proliferate, and differentiate into plasma cells. The antibody produced is predominantly IgM, present at relatively low titer, of lower average affinity, and short-lived.

The secondary (anamnestic) response occurs on re-exposure to the same antigen (or a cross-reacting epitope) and is faster, larger, and qualitatively different. Memory B cells generated during the primary response respond within 1-3 days, antibody titer rises higher and persists longer, and the predominant class switches to IgG with progressively higher affinity through somatic hypermutation and affinity maturation in germinal centers. This is precisely why a patient with a prior undetected alloantibody (now below the threshold of detection, or "evanescent") can mount a rapid, high-titer IgG response and a delayed hemolytic transfusion reaction days after a unit that initially crossmatched compatible.

B Cells, T Cells, and Macrophages

B lymphocytes originate and mature in the bone marrow (the "B" is a historical reference to the avian Bursa of Fabricius, but in mammals maturation is marrow-based). Each B cell displays a unique surface immunoglobulin that serves as its antigen receptor; upon activation it differentiates into an antibody-secreting plasma cell or a long-lived memory B cell.

T lymphocytes also originate in the bone marrow but mature in the thymus, where they undergo positive and negative selection against self-antigen. Two functional subsets matter most for the SBB exam: CD4+ helper T cells, which recognize antigen presented on MHC class II and secrete cytokines that direct B-cell class switching and macrophage activation, and CD8+ cytotoxic T cells, which recognize antigen on MHC class I and directly kill infected or foreign cells. A third subset, regulatory T cells (Tregs), suppresses excessive immune activation and helps maintain self-tolerance.

Antigens are classified by their T-cell dependence. T-dependent antigens -- typically protein antigens -- require helper-T-cell cooperation with B cells; this cooperation is what enables isotype (class) switching to IgG, affinity maturation, and memory-cell formation. T-independent antigens -- typically repetitive polysaccharide epitopes, such as many blood group carbohydrate antigens -- can activate B cells directly by cross-linking many surface Ig receptors simultaneously, without T-cell help. The response to T-independent antigens is predominantly IgM, without memory or affinity maturation. This distinction explains a recurring exam pattern: naturally occurring antibodies against carbohydrate blood group antigens (ABO, and some Lewis and P1 specificities) behave as classic T-independent, IgM-dominant responses, while antibodies against protein-based Rh, Kell, Duffy, and Kidd antigens behave as T-dependent responses capable of the IgG class switch and anamnestic boosting that make them clinically dangerous on re-exposure.

Macrophages are phagocytic cells derived from circulating monocytes that take up residence in tissue (Kupffer cells in the liver, splenic macrophages, and others -- collectively the mononuclear phagocyte system). They perform three roles central to blood banking: (1) phagocytosis of opsonized particles, including IgG- or C3b-coated red cells, which is the mechanism of extravascular hemolysis; (2) antigen presentation, processing engulfed antigen and displaying peptide fragments on MHC class II to activate CD4+ T cells; and (3) cytokine secretion (IL-1, IL-6, TNF-alpha) that drives fever and the acute-phase response -- the basis of febrile nonhemolytic reactions.

The Genetic Basis: HLA and Antibody Diversity

The major histocompatibility complex (MHC), called HLA (human leukocyte antigen) in humans, is encoded on the short arm of chromosome 6. Class I HLA (HLA-A, -B, -C) is expressed on essentially all nucleated cells and platelets, presents endogenous (intracellular) peptide to CD8+ T cells, and is the antigen system responsible for platelet refractoriness, HLA alloimmunization from transfusion or pregnancy, and graft rejection. Class II HLA (HLA-DR, -DQ, -DP) is restricted to antigen-presenting cells (macrophages, dendritic cells, B cells) and presents exogenous peptide to CD4+ T cells; Class II mismatch drives transfusion-associated graft-versus-host disease (TA-GVHD) risk and is central to hematopoietic stem cell donor matching.

Antibody diversity itself is generated genetically through V(D)J recombination: variable (V), diversity (D), and joining (J) gene segments are randomly recombined in developing B cells, and combined with junctional diversity and somatic hypermutation, this process generates an antibody repertoire large enough to recognize virtually any antigen -- the molecular basis of clonal selection theory, in which a pre-existing repertoire of antigen-specific clones is expanded, not created anew, by antigen exposure.