There are five major classes of immunoglobulins - IgG, M, A, D and E. Each is made of two heavy and two light chains or polymers of this basic subunit. The structure of each heavy and light chain consists of a variable region involved in antigen specificity and binding and a constant region which is involved in the class specific properties. The different classes of immunoglobulins differ in the biological properties. Of particular interest is the IgA fraction which provides antibodies in all the external secretions - including the reproductive organs. A few speculations are offered as to the possible role of IgA in reproductive biology.