The activation of T cells often requires more than the simple binding of the T cell antigen receptor to a foreign protein bound to MHC molecules. A number of other cell surface molecules on T cells and antigen-presenting cells are usually actively involved in T cell activation. Of particular importance functionally are the interactions of two cell surface glycoproteins, termed CD4 and CD8, the expression of which on almost all mature T cells is mutually exclusive. These molecules function to target the T cell antigen receptor to class II or class I molecules. This is achieved by the direct interaction of peptide-bearing class II and I molecules with CD4 and CD8 molecules, respectively.
The expression of CD4 or CD8 molecules on T cells is coordinately regulated with T cell function. The primary function of TCD8+ is to limit the replication of influenza virus. This is achieved by destroying virus-infected cells, and, possibly, by releasing lymphokines with antiviral activity. The primary function of CD4-expressing cells (Tcd4+) is to enhance antibody responses and specific and nonspecific cellular immunity. Consistent with these functions, class I molecules are expressed by virtually all cell types, while class II molecules are expressed largely by B cells and inflammatory cells (monocytes, macrophages). Upon exposure to any of several cytokines (most importantly interferon y and tumor necrosis factor a), class II- molecules are induced in many cell types, enabling TCD4+ to participate more directly in exerting antiviral effector functions.
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