The antibodies produced by B cells are a type of globulin protein called immunoglobulins. Antibody molecules are able to chemically recognize surface portions, or epitopes, of large molecules that act as antigens, such as nucleic acids, proteins, and polysaccharides. About 10 amino acid subunits of a protein may compose a single epitope recognizable to a specific antibody. The fit of an epitope to a specific antibody is analogous to the way a key fits a specific lock. All antibody molecules are alike except for certain small segments that, varying in amino acid sequence, account for the specificity of the molecules for particular antigens.
Antibodies act in several ways.
Antibodies attached to antigens on the surfaces of invading cells activate a group of at least 11 blood serum proteins called complement, which cause the breakdown of the invading cells in a complex series of enzymatic reactions. Complement proteins are believed to cause swelling and eventual rupture of cells by making holes in the lipid portion of the cell's membrane. The T cells interact with the body's own cells, regulating the immune response and acting against foreign cells that are not susceptible to antibodies in what is termed cell-mediated immunity. Each T cell has certain membrane glycoproteins on its surface that determine the cell's function and its specificity for antigens. Another type of membrane glycoprotein is the receptor that helps the T cell recognize the body's own cells and any foreign antigens on those cells.