Immune Cellular Function : Lozol
Posted by Surgery on Jul 11, 2008
T Cells
Two of the main functions of T cells are (1) to provide cytotoxic activity against facultative intracellular pathogens (mycobacteria, fungi) and virally infected cells or tumor cells, and (2) to provide cytokines (Table 15-2) to help B cells make antibodies.
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For the T cell to carry out these functions, it first must bind to the target cell or to the antigen-presenting cell (APC), respectively. For high-affinity binding of T cells to target cells or APCs, several molecules on T cells in addition to TCRs bind to molecules on their respective target cells (Fig. 15-2).
CD8 molecules on cytotoxic T cells bind to MHC class I molecules on target cells. CD4 molecules on helper T cells bind to MHC class II molecules on APCs. Lymphocyte function antigen 1 (LFA-1: Lozol ) molecules on T cells bind to intracellular adhesion molecule 1 (ICAM-1 or CD54) on APCs. CD2 molecules on T cells bind to LFA-3 or CD58 on APCs. With the adhesion of cytotoxic T cells to their targets, they are stimulated to kill the target; and with the adhesion of helper T cells to APCs, they are stimulated to produce cytokines, which stimulate B cells and other cells. Many other adhesion pairs participate in these important cell“cell interactions.
B Cells
In the primary antibody response, native antigen is carried to a lymph node draining the site of entry, taken up by specialized cells called follicle-stimulating cells (FSCs: Lozol ), and expressed on their surfaces. Virgin B cells bearing surface immunoglobulin specific for that antigen then bind to the antigen; if the affinity of the B-cell surface immunoglobulin for the antigen present on the FSCs is high, and if other signals are provided by activated helper T cells, the B cell develops into an antibody-producing plasma cell. If the affinity is not high enough, or if T-cell signals are not received, the B cell dies by apoptosis. The signals provided by activated helper T cells include those from cytokines they secrete (IL-4, IL-5, IL-6, IL-10, and IL-13) (1: Lozol); (Table 15-2: Lozol) and the signal from a surface T-cell molecule, CD154, which binds to CD40 on the B-cell surface (5) (Fig. 15-2: Lozol ). Cross-linking of CD40 on B cells or allowing CD40 to interact with CD154 in the presence of certain cytokines causes the B cells to undergo proliferation and to initiate immunoglobulin synthesis. In the primary immune response, usually only IgM antibody is made, and most of it is of relatively low affinity. Some B cells become memory B cells during the primary immune response. These cells have switched their immunoglobulin genes so IgG, IgA (Lozol), or IgE antibodies of higher affinity are formed on a secondary exposure to the same antigen. The secondary immune response occurs when these memory B cells again encounter that antigen. Plasma cells form, just as in the primary response; however, many more cells are rapidly generated, and IgG, IgA, and IgE antibodies of increased affinity are produced (Lozol ). The exact pattern of isotype response to antigen varies, depending on the type of antigen and the cytokines present in the microenvironment.
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