Antigens and Antibodies
Introduction
This domain defines antigens, immunogens, epitopes, haptens, and antibody structure and function. The exam tests immunoglobulin classes, affinity versus avidity, and the behaviour of antibodies in precipitation and agglutination.
Core rules
- An antigen is a molecule recognised by an antibody or T-cell receptor; an immunogen is an antigen that can induce an immune response.
- An epitope is the specific binding site; a hapten is a small molecule that is not immunogenic alone but becomes immunogenic when coupled to a carrier.
- Antibodies are Y-shaped molecules with two heavy and two light chains, variable antigen-binding Fab regions, and a constant Fc region.
- IgG is the main circulating antibody and crosses the placenta; IgM is a pentamer and appears early; IgA is secreted as a dimer; IgE binds mast cells; IgD is mainly on naive B cells.
- Affinity is strength of one antigen-antibody bond; avidity is overall strength of multivalent binding.
Worked example
Penicillin alone is too small to induce an immune response, so it acts as a hapten. When penicillin metabolites bind covalently to a carrier protein, the conjugate can stimulate B cells and T cells. Later antibody can bind the penicillin epitope, which explains some penicillin allergy reactions. The hapten is the epitope-bearing small molecule, not the whole immunogenic conjugate.
Traps
Not every antigen is immunogenic. Excess antibody can cause a prozone false negative in agglutination. IgM is pentameric and can show high avidity even when individual binding sites have modest affinity.