Antibodies are produced by plasma cells. Fig. 6.1 shows antigens bound to antigen-binding sites of an antibody molecule. [Figure 6.1] It is estimated that the immune system of each person can make enough antibodies to bind to over 10¹² different antigens. When plasma cells make antibody molecules they combine the polypeptides produced by the expression of genes for heavy chains and the genes for light chains. Research has shown that producing this very large number of antibodies is only possible by modifying the primary transcripts of the genes that code for heavy chains and the genes that code for light chains.