After first exposure to antigen, mature B cells in a resting state proliferate and differentiate into antibody-producing plasma cells, which synthesize the soluble form of the immunoglobulin receptor. In serum and other body fluids, antibody production is primarily derived from plasma cells. After the first exposure of antigen-activated B cells to the antigen (initial response), antigen-activated B cells differentiate to produce soluble IgM. This initial activation requires the “help” of T lymphocytes (discussed below) that activate the B cells. After activation, some B cells will change the class of immunoglobulin from IgM to another form (IgG, IgA, etc.). This phenomenon is known as immunoglobulin class switching and involves the recombination of a segment of DNA in which the genetic information encoding the V region of the antigen-binding heavy chain is misaligned with that encoding the C region of a different heavy chain, resulting in a new heavy chain that has the same antigen-binding characteristics of the original IgM (same V region), but with different Immunological properties. Upon exposure to antigen, some B cells differentiate into antibody-producing plasma cells and some into memory B cells. These memory B cells are long-lived in order to recognize the antigen and produce a rapid immune response when the body is re-exposed to the antigen. These memory B cells are responsible for the long-lasting effect of the vaccine. T lymphocytes T lymphocytes are another important population of lymphocytes in the adaptive immune system. t cell precursors are derived from the bone marrow and develop into mature T cells in the thymus. Mature T cells secrete and produce soluble cytokines, which are important mediators. t cells have many important functions, including the activation of helper B cells and the production of cytotoxic effects against cells infected by pathogens and tumor cells. T cells express surface antigen receptors similar to immunoglobulins, but differ in some properties. First, T cell surface antigen receptors consist of disulfide-linked α and β chains, each with an amino-terminal V region and a carboxy-terminal C region, which, like immunoglobulins, are associated with antigen recognition. However, these recombinant gene fragments are separate and distinct. Also similar to B cells, this genetic reorganization occurs during T cell development in the thymus, but T cell antigen receptors do not undergo class switching and do not secrete soluble antigen receptors. Antigen recognition by T cells differs from that of B cells in that, first of all, the combination of natural antigens enables the activation of B cells, but T cells require the presentation of antigens on their surface by other cell types in order to be recognized and activated. These cells are called antigen-presenting cells (APCs), and several types of cells can act as APCs, including dendritic cells, macrophages, and B cells. Second, T cells do not recognize natural antigens; they recognize only short peptides of 8-20 amino acids, which are produced by the presentation of the original antigen by the APC. Within the APC, the antigen is degraded by protein hydrolases into multiple small fragments of short peptides that bind to the major histocompatibility complex (MHC) on the surface of the APC and are transported to the surface of the APC, a complex group of genes that encode the protein product that presents the antigenic short peptide to the T cell. Antigenic short peptides are recognized by antigen receptors on the surface of T cells only after they bind to MHC molecules in a complex. Because of this specific recognition feature, the recognition process of T cells becomes dual specific or MHC-restricted.