Specific immune responses are directed against antigens.
Antigens are molecules or parts of molecules that are usually large, complex, and foreign.
A given molecule can have a number of antigenic determinant sites that stimulate the production of different antibodies. Specific immunity is a function of lymphocytes. These subclasses differ in regard to the polypeptides in the constant region of the heavy chains. The combination of antibodies with antigens promotes phagocytosis. Antigen-antibody complexes activate a system of proteins called the complement system.