Antibody Types And Reactions

Ab types and reactions are classified on the basis of variations in a common section of the Fc fragment that governs biological activity in a general way.

IgG (Fc = y) (Fig. 5.5) participates in precipitation reactions, toxin neutralizations, and complement fixation. IgG is the major (70%) human Ig. The Fab tip fixes antigen, and the Fc fragment can fix complement to yield agglutination or lysis. IgG is the only immunoglobulin that crosses the transplacental barrier and the neonatal stomach, so it provides maternal protection. IgG constitutes about 75% of the total Ab in the circulation. It is present at a concentration of about 15 mg/mL and has a half-life of 3 weeks, the longest of any of the Ab types. The light chains of IgG can possess either k or A variants. These slight differences in structure are called isotypes, and the phenomenon is termed isotypic variation.

IgM (Fc = f) (Fig. 5.6) is present at a concentration of about 1.5 mg/mL and has a half-life of less than 1 week. This Ab participates in opsonization, agglutination reactions, and complement fixation. Opsonization, as stated previously, is a "protein coating" or tagging of a bacterium that renders it more susceptible to phagocytosis. A complex of the Fc portion of IgM plus C3b of complement is that protein. IgM is the first immunoglobulin formed during immunization, but it wanes and gives way to IgG. IgM is a pentamer, and its agglutination potency is about 1,000 times that of IgG. IgM is also responsible for the A, B, and O blood groups. The fundamental monomeric IgM structure is much like that of IgG. The pentamer is held together by disulfide bonds and a single J (joining) peptide. The affinity of an IgM monomer for antigen is less than that of IgG, but the multimeric structure raises the avidity of the molecule for an antigen.

Figure 5.9 • Structure of immunoglobulin A (IgA), the mucosal Ab that protects the GI tract and the respiratory mucosa.

Membrane Transport Receptor

Figure 5.9 • Structure of immunoglobulin A (IgA), the mucosal Ab that protects the GI tract and the respiratory mucosa.

IgA (Fc = a) (Fig. 5.9) is found in exocrine gland secretions (milk, saliva, tears), where it protects mucous membranes (e.g., in the respiratory tract). It is present in the serum as a monomer at a concentration of 1 to 2 mg/mL, but humans secrete about 1 g of the dimer per day in the mucosal fluids. Secretory IgA consists of two IgG-like units linked together at the Fc regions by a peptide known as the secretory fragment and a J fragment. The secretory fragment is actually part of the membrane receptor for IgA. The IgA molecule on the mucosal side of the membrane binds antigen, then binds to the receptor. By a process of transcytosis, the IgA-antigen complex is moved from the mucosa to the bloodstream, where IgG and IgM can react. Because it is distributed on the mucosa, IgA has an anatomically specific distribution, unlike the other antibodies. IgA is the mediator of oral polio vaccination (the mucosal reaction gives way to systemic protection).

IgD (Fc = 8) is present on the surface of B cells and, along with monomeric surface IgM, is an antigen receptor that activates Ig production. There is less than 0.1 mg/mL in the bloodstream, and the half-life is only 3 days.

IgE (Fc = s) is the Ab responsible for hypersensitivity reactions. IgE complexes have a high affinity for host cell surfaces and can damage the host. High levels of IgE are found in persons with allergies of various types, as well as in autoimmune diseases. The Fc fragment is responsible for the Ig-cell reactivity. An Ab-plus-antigen reaction yields the typical Ab-antigen complex. The Fc portion of the Ab is actually part of the mast cell. When antigen binds to the Fab portion of the Ab, the IgE molecules become cross-linked. This probably distorts the membrane of the mast cells and stimulates them to release histamine, which causes bronchial constriction, itching, redness, and anaphylaxis.

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