FIGURE 14.4 • Schematic diagram of the molecular structure of MHC I and MHC II molecules. The MHC I molecule is a glycoprotein that is expressed on the surface of all nucleated cells of the body and on platelets. MHC I molecules present endogenously synthesized peptides for recognition by cytotoxic CD8 T lymphocytes. Therefore, the MHC I molecule acts as the target for the elimination of abnormal host cells producing abnormal proteins (e.g., cells infected by an intracellular agent such as a virus, or cells that have been transformed such as cancer cells). MHC I consists of an  heavy chain (45 kilodaltons) and a smaller, noncovalently attached 2 microglobulin polypeptide (12 kilodaltons). The 2 microglobulin promotes maturation of T cells and acts as a chemotactic factor. The MHC II molecule is also a glycoprotein but is expressed only on a restricted population of cells known as antigen-presenting cells (APCs). MHC II molecules present exogenous (foreign) peptides to helper CD4 T lymphocytes. They consist of two chains–an  chain (33 kilodaltons) and a  chain (29 kilodaltons)—each of which possesses oligosaccharide groups.
the CTL also releases cytokines that stimulate cells to proliferate and destroy the abnormal host cells.
CD8 T lymphocytes are MHC I restricted, and CD4 T lymphocytes are MHC II restricted.
MHC molecules are recognized by helper CD4 T lymphocytes or CTLs, depending on the class of the MHC molecule engaged. This restricted presentation of foreign antigens by MHC molecules to either cytotoxic or helper T lymphocytes is a key component of immune surveillance.
The MHC I molecule with the peptide antigen displayed on its surface interacts only with the TCR and CD8 molecule expressed on cytotoxic CD8 T lymphocytes; these cells are therefore described as MHC I restricted. This interaction allows cytotoxic T lymphocytes to recognize infected or transformed target cells (Fig. 14.5a).
In contrast, the MHC II molecule with the peptide antigen displayed on its surface interacts only with the TCR and CD4 molecule expressed on helper CD4 T lymphocytes (Fig. 14.5b); these cells are therefore described as MHC II restricted. MHC II molecules are found on APCs, such as macrophages, whose main function is to present antigen to T lymphocytes.
For B cells to become activated and differentiate into plasma cells, they require interactions with helper  T lymphocytes.
Each B lymphocyte reacts only with a single antigen or type of antigenic site that it has been genetically programmed to recognize. Activation of B cells requires two signals. One is derived from interaction between BCRs and antigen. The bound antigen molecules are engulfed into B cells by  receptor-mediated endocytosis, and fragments of the antigen are then displayed at the cell surface with the help of MHC II molecules. Helper T cells with complementary TCRs bind B cells and provide the second costimulatory signal. The binding usually involves a reaction between CD40 molecules on a B-cell surface with their ligands (CD40L or CD154) residing at the surface of a helper T cell. These interactions complete the activation process of a B lymphocyte and induce an involved T cell to secrete specific cytokines that stimulate divisions and differentiation of B cells. Details of B-cell activation are illustrated in Figure 14.6.