Immunoglobulin is a key factor of the immune response resulting from B-cell activation and associated with T-cell stimulation. Because of its structure, this antibody has a dual function: it specifically recognizes the inducer antigen in the variable region and eliminates it by a constant portion which is responsible for effector properties. Surface immunoglobulin, therefore, is the B-cell antigen receptor; it differs from the T-cell receptor in that it recognizes the antigen unbound to the major istocompatibility complex; binding the antigen results in direct signal transduction first in the cytoplasm, then in the nucleus. This receptor can be secreted in the body: it is made up of circulating immunoglobulins. Human immunoglobulins are divided into 5 classes, each of them with its own response kinetics, distribution and functions. The variability of the antibody response accounts for a genetic organization involving numerous genes which may be associated with each other, or mutate, or recombine during maturation of the lymphocytes. Altogether, this system has a theoretical capacity of response to three hundred million different antigens.