U21 from some human herpesvirus 7 binds and targets certain MHC I molecules for lysosomal degradation. E19 from some adenoviruses block the movement of MHC I to the proper locations for the endogenous pathway. Nef from some HIV strains enhance the movement of MHC molecules back into the cytoplasm, preventing them from presenting antigens. The role of Langerhans' dendritic cells in antigen processing Langerhans' cells are particular type of dendritic cells present in non lymphoid tissues together with interstitial cells. When these cells (in an immature state) come in contact with antigenic cells or disease causing viruses etc. these cells produce an inflammatory stimulus and start antigen processing and move toward lymph nodes where these APCs present antigen to mature T lymphocytes. T-dependent antigen – Antigens that require the assistance of T cells to induce the formation of specific antibodies. T-independent antigen – Antigens that stimulate B cells directly. B-cell activation with B-T cell interactions Lymphocytes are one of the five kinds of white blood cells or leukocytes, circulating in the blood. Although mature lymphocytes all look pretty much alike, they are diverse in their functions. The most abundant lymphocytes are: B lymphocytes (often simply called B cells) T lymphocytes (likewise called T cells) B cells are produced in the bone marrow. The precursors of T cells are also produced in the bone marrow but leave the bone marrow and mature in the thymus (which accounts for their designation). Each B cell and T cell is specific for a particular antigen, which simply means that each of these cells is able to bind to a particular molecular structure (such as an antigen). The specificity of binding resides in a specific receptor for antigen: the B-cell receptor (BCR) and the T-cell receptor (TCR) for B and T cells, respectively. Both BCRs and TCRs share these properties: They are integral membrane proteins. They are present in thousands of identical copies exposed at the cell surface. They are made before the cell ever encounters an antigen. They are encoded by genes assembled by the recombination of segments of DNA. How antigen receptor diversity is generated Each receptor has a unique binding site. This site binds to a portion of the antigen called an antigenic determinant or epitope. The binding, like that between an enzyme and its substrate, depends on complementarity of the surface of the receptor and the surface of the epitope and