Immune System and Antibodies

Leukocyte types

Leukocytes are classified by whether their cytoplasm contains visible specific granules. Granular leukocytes include neutrophils, eosinophils and basophils, and mast cells, which are tissue-resident cells filled with histamine-containing granules, are usually grouped with them. Agranular leukocytes lack visible specific granules and comprise lymphocytes (T cells and B cells) and monocytes. Therefore, when agranular cells are identified, T cells and monocytes are the correct answers, and eosinophils and mast cells are not.

The cells are also distinguished by their functions, which are often tested as matching pairs. Neutrophils are the first leukocytes recruited to a site of infection, where they phagocytose microbes. B cells differentiate into plasma cells that secrete antibody. Natural killer cells kill antibody-coated target cells by antibody-dependent cell-mediated cytotoxicity, in which the Fc region of bound IgG engages Fc receptors on the NK cell. Mast cells release histamine from their granules during immune and allergic responses.

Example: A cell whose cytoplasm is packed with histamine-containing granules is a mast cell, not a monocyte, because monocytes are agranular and mature into macrophages in tissues.

Immunoglobulin classes

Immunoglobulins are grouped into five classes, IgG, IgM, IgA, IgE and IgD, according to the heavy chain constant region (gamma, mu, alpha, epsilon and delta). The class determines effector function, tissue distribution and the stage of the immune response in which the antibody appears.

IgA is the principal antibody of mucosal secretions. In serum it is mostly monomeric, whereas secretory IgA is a dimer linked by the J chain. Dimeric IgA binds the polymeric immunoglobulin receptor on the basolateral surface of epithelial cells, is carried across the cell by transcytosis, and is released into secretions. Because secretory IgA is abundant in colostrum and milk, it protects the newborn at mucosal surfaces.

IgE is the class responsible for immediate hypersensitivity. Its Fc region binds the high-affinity receptor Fc\varepsilonRI on mast cells and basophils. Cross-linking of this bound IgE by an allergen triggers degranulation and the release of histamine and other mediators, and this mechanism underlies allergy and allergic asthma.

IgM is the first antibody produced in a primary response and circulates mainly as a pentamer. IgD is chiefly a surface receptor on naive B cells. Neither of these classes is the main mediator of allergy.

Example: In a patient whose asthma is triggered by pollen and whose mast cells carry allergen-specific antibody, the antibody responsible is IgE rather than IgA.

HAT selection in hybridoma cells

Hybridoma technology fuses antibody-producing B cells with myeloma cells, and the mixture is grown in HAT medium (hypoxanthine, aminopterin and thymidine). Aminopterin blocks de novo synthesis of purines and pyrimidines, so cells must rely on the salvage pathway. In this pathway, hypoxanthine-guanine phosphoribosyltransferase (HGPRT) converts hypoxanthine and guanine to IMP and GMP.

Myeloma cells lack HGPRT, so they cannot salvage purines and die in HAT medium. This deficiency is the basis of selection. Unfused B cells also disappear because they have a limited lifespan in culture. Only hybrids that inherit a functional HGPRT gene from the B cell partner survive and can be cloned. Normal B cells, T cells and macrophages all express HGPRT and can therefore recycle purines through the salvage pathway.

Example: An HGPRT-deficient myeloma line is chosen as the fusion partner so that unfused myeloma cells are eliminated by HAT, leaving only B cell-myeloma hybrids.

ABO blood group antibodies

The ABO system is determined by the A and B antigens present on red blood cells. Group A carries the A antigen, group B carries the B antigen, group AB carries both, and group O carries neither. Individuals make naturally occurring antibodies, mostly IgM, against the antigen they lack.

• Group A: anti-B antibodies in serum
• Group B: anti-A antibodies in serum
• Group AB: neither anti-A nor anti-B
• Group O: both anti-A and anti-B

Anti-B antibodies are therefore present in the serum of blood group A and blood group O individuals, who both lack the B antigen. A serum sample from a group A donor will agglutinate group B red cells because it contains anti-B.

Group O red cells lack both A and B antigens, which is why they are used as universal red cell donors.

Herd immunity

Herd immunity (community immunity) is the indirect protection of susceptible individuals that arises when a large enough fraction of a population is immune, either through infection or vaccination. Because few infected people are present to transmit the pathogen, the probability that a susceptible person meets it falls.

The threshold immune fraction is p_c = 1 - \frac{1}{R_0}, where R_0 is the basic reproduction number. For a pathogen with R_0 = 4, about 75 percent of the population must be immune to stop sustained transmission.

This population-level concept should be distinguished from individual-level terms. Innate immunity is nonspecific and present before any exposure. Adaptive immunity is antigen-specific and mediated by an individual's lymphocytes. Active immunity is the protection an individual develops through its own immune response after infection or vaccination.

Example: Vaccinating most of a community can protect an unvaccinated infant, even though the infant mounts no immune response of its own.

Membrane vs secreted immunoglobulin forms

A single B cell makes a membrane-bound B cell receptor (BCR) and, after differentiation, secreted antibody. Both forms contain the same rearranged variable region, which was generated once by V(D)J recombination. They therefore share an identical antigen-binding site and recognize the same epitope. The surface IgM and the IgM secreted by the same cell bind the same antigen.

Both forms are encoded by one immunoglobulin heavy chain gene. The membrane form has extra C-terminal exons that encode a hydrophobic transmembrane anchor and a short cytoplasmic tail, whereas the secreted form ends in hydrophilic exons. The difference arises from alternative RNA processing of one primary transcript: differential splicing selects membrane or secretory exons, and the choice of polyadenylation site determines the transcript end. The two forms are therefore not encoded by different genes.

The BCR associates with the signalling subunits Ig-alpha and Ig-beta, which are absent from secreted antibody.

Example: A statement that the surface receptor and the secreted antibody of one B cell have different epitope specificity is false, because their antigen-binding sites are identical.