Antibodies

In mammals there are five types of antibody: IgA, IgD, IgE, IgG, and IgM, with 4 IgG and 2 IgA subtypes present in humans. (Ig stands for immunoglobulin, which is another name for antibody). These are classified according to differences in their heavy chain constant domains (see below for more information regarding the structural features of antibodies). Each immunoglobulin class differs in its biological properties and has evolved to deal with different antigens. IgA can be found in areas containing mucus (e.g. in the gut, in the respiratory tract or in the urinogenital tract) and prevents the colonization of mucosal areas by pathogens. IgD functions mainly as an antigen receptor on B cells. IgE binds to allergens and triggers histamine release from mast cells (the underlying mechanism of allergy) and also provides protection against helminths (worms). IgG (in its four forms) provides the majority of antibody-based immunity against invading pathogens. IgM is expressed on the surface of B cells and also in a secreted form with very high affinity for eliminating pathogens in the early stages of B cell mediated immunity (i.e. before there is sufficient IgG to do the job).
Immature B cells express only IgM on their cell surface (this is the surface bound form not the secreted form of immunoglobulin). Once the naive B cell reaches maturity, it can express both IgM and IgD on its surface - it is the co-expression of both these immunoglobulin isotypes that renders the B cell 'mature' and ready to respond to antigen. Following an engagement of the immunoglobulin molecule with an antigen, the B cell becomes activated, and begins to divide and differentiate into an antibody producing cell (sometimes called a plasma cell). In this activated form, the B cell will produce its immunoglobulin in a secreted form rather than a membrane-bound form. Some of the daughter cells of the activated B cells will undergo isotype switching, a mechanism by which the B cell begins to express the other heavy chains and thus produce IgD, IgA or (more commonly) IgG.