Antigen Recognition and Antibodies

Study Notes

Antibodies are immunoglobulin molecules with specificity for an epitope of antigen molecules, synthesized by B lymphocytes (B cells) and secreted by plasma cells.
Antibodies provide protection by neutralizing viruses or toxins, enhancing phagocytosis and leukocyte degranulation, and activating complement activation with subsequent lysis.

Human immunoglobulin contains four polypeptides: two identical light chains and two identical heavy chains linked by disulfide bonds.
Antibody molecules share the same basic structural characteristics but display remarkable variability in the regions that bind antigens.
Heavy chains and light chains consist of amino-terminal variable (V) regions that participate in antigen recognition and carboxy-terminal constant (C) regions that mediate effector functions.

Hypervariable regions of VH and VL are responsible for antigen binding by antibody molecules.
Three short stretches in the V region of the heavy chain and three stretches in the V region of the light chain exhibit the greatest diversity, known as hypervariable regions.

Antibody molecules can be divided into distinct classes and subclasses based on differences in the structure of their heavy chain C regions.
Classes of antibody molecules, also called isotypes, are named IgA, IgD, IgE, IgG, and IgM, each performing different effector functions.

B cells initially express only IgD and IgM antibodies, and the class is determined by the constant region (Fc) of the heavy chain.
B cells can switch to one of the other classes of antibody (IgA, IgE, or IgG) by replacing the tail region of the antibody in response to chemical messengers (cytokines) from T helper cells.

Antibodies recognize and bind antigen, and promote the killing and/or removal of the immune complex through activation of effector mechanisms.

There are three types of immunoglobulin variability: isotypic variation (present in the germline of all members of a species), allotypic variation (intraspecies allelic variability), and idiotypic variation (diversity at the antigen-binding site).

Primary response: Lag phase (5-10 days), peak concentration (relatively low), Ig class (IgM – early response then IgG), average antibody affinity (relatively low), and responding cell (resting B cell).
Secondary response: Lag phase (2-5 days), peak concentration (relatively high), Ig class (IgG predominates), average antibody affinity (relatively high), and responding cell (memory B cell).