Antigen is any substance that may be specifically bound by an antibody molecule. The antigens are mostly proteins, but can be polysaccharides, glycoproteins or other types of conjugated proteins, nucleic acid or such chemicals. Each antigen has many antigenic determinants (called epitopes) of the antigen. As each antibody binds with 2 or more such determinants, the one antigen can bind with many antibodies. In order to generate antibodies specific for small molecules, the latter are attached to a larger molecule before immunization. The number of different antigens recognized by lymphocytes in the body is extremely large. The recognition ability is innate and develops without exposure to the antigen. Stem cells differentiate into many million different \(\mathrm{T}\) and \(\mathrm{B}\) lymphocytes, each with the ability to respond to a particular antigen. When the antigen first enters the body, it can bind directly to the appropriate receptors on B cells. However, a full antibody response requires that the B cells contact helper \(T\) cells. In the case of \(T\) cells, the antigen is taken up by an antigen-presenting cells and partially digested. A peptide fragment of it is presented to the appropriate receptors on \(\mathrm{T}\) cells. In either case, the cells are stimulated to divide, forming clones of cells that respond to this antigen.