Cellular Immune Response

Cellular Immune Response

T cells attach to foreign, antigen-bearing cells such as bacterial­ cells and interact with direct cell-to-cell con-tact. This is known as cellular immune response or cell-mediated immunity. T cells, along with some mac-rophages, also synthesize polypeptides called cytokines that enhance responses to antigens. Interleukin­-1 and interleukin-2 stimulate synthesis of cytokines fromother T cells. Other cytokines, called colony-stimulatingfactors, stimulate leukocyte production in red bonemarrow, activate macrophages, and cause B cells to grow. In the cellular immune response, lympho-cytes themselves defend the body. The targets of cell-­ mediated immunity are cancer cells, cells of foreign grafts, and tissue cells infected with viruses or parasites. Lymphocytes either directly kill infected cells or cause their death indirectly by releasing their chemicals that enhance inflammation or activate other lymphocytes or macrophages. Both B and T lymphocytes acquire immunocompetence and self-tolerance in different locations in the body.

Certain activated T cells kill body cells that are infected by bacteria or viruses directly. They also may directly kill cells that are cancerous or abnormal and cells from foreign tissues that have been transplanted or infused. Different T cells release chemicals that reg-ulate the immune response. T cells that mediate cellu-lar immunity are much more complicated in function and classification that B cells. The two primary types of T cells are identified by cell differentiation glycopro-teins known as CD4 or CD8. Both are surface recep-tors that are different from T cell antigen receptors and help with interactions between other cells and T cells. Activation of CD4 and CD8 cells differentiate them into three major types of effector cells involved in cellular immunity. CD4 cells usually become helperT cells, and activate B cells, other T cells, and macro-phages. They direct the adaptive immune response. CD8 cells become cytotoxic T cells, destroying cells in the body that contain anything foreign. Some CD4 cells become regulatory T cells, which regulate the immune response. Activated CD4 and CD8 cells can also become memory T cells. All T cells have a CD3 receptor complex in the plasma membranes thateventually activates them to recognize antigens.

Antigen-Presenting Cells

Before it can respond to an antigen, a lymphocyte must be activated. T cells are activated by the presence of processed antigen fragments attached to the surface of an antigen-presenting cell, also called an accessorycell, which may be macrophages, B cells, or other typesof cells. Antigen-presenting cells have important aux-iliary functions in regard to antigens and respond to them differently than lymphocytes do. When a mac-rophage phagocytizes a bacterium and digests it in its lysosomes, T-cell activation begins. Some bacterial antigens then move to the surface of the macrophage. They are displayed near certain protein molecules that make up the major histocompatibility complex (MHC). MHC antigens help T cells recognize foreign antigens. Helper T cells contact displayed foreign anti-gens. If the antigen combines with the helper T cell’s antigen receptors, it becomes activated and stimulates a B cell to produce antibodies specific for the displayed antigen. The membrane glycoproteins formed as part of the MHC are known as MHC proteins or humanleukocyte antigens.

Class I MHC proteins are found in the plasmamembrane of every nucleated cell. Their activation eventually results in destruction of abnormal cells, which is a significant component why donated organs are often rejected. Class II MHC proteins are only found in plasma membranes of antigen-presenting cells and lymphocytes. Phagocytic antigen-presenting cells break down and engulf foreign antigens or pathogens This antigen processing creates antigen fragments, which become bound to class II MHC ­proteins and then inserted into the plasma membrane. The class II MHC proteins only appear in the plasma mem-brane when the cell is processing antigens. TABLE 20-4 summarizes biochemical and functional characteris-tics of B cells and T cells.