helper T cell

Helper T cells, T helper cells (Th), effector T cells circulate throughout the body where they interface with MHC class II protein on other cells, determining whether the MHC class II is presenting 'self' or 'non-self protein' (antigen). MHC class II receptors are located on the surface of professional antigen presenting cells (APCs), which display epitope proteins – exogenous antigen or fragmented angtigen from phagocytosed cells – on their surfaces.

When a helper T cell is activated by contact with antigen, it enters the cell cycle in addition to producing lymphokines and chemokines. Th cells direct antibody class switching in B lymphocytes, orchestrate activation and growth of cytotoxic T cells, and maximize the bactericidal activity of phagocytes (macrophages).

Naïve B lymphocytes each have one of millions of distinct surface antigen-specific surface receptors, yet have not encountered their specific, cognate antigen. With a life-span of only a few days, many B cells die without ever encountering their cognate antigen. Naïve B cells are stimulated when the BCR binds to its cognate antigen. This antigen-Ig binding must be coupled with a signal from a helper T cell in order to activate the B cell.

Helper T cells mostly carry the CD4 surface protein, though a few carry CD8. The CD4 receptor triggers targetting by HIV, which determines the crippling effect of HIV on the immune system.
Subsets of Th cells are defined by the class of cytokine that they secrete upon activation:
Th1 – produce copious amounts of IL-2 and IFN-γ.
Th2 – particularly effective at stimulating B cells through secretion of IL-4, IL-5, and IL-6.
Th3 – produce cytokine transforming growth factor-beta (TGF-β) and IL-10.

Th1 cells are the more effective antiviral agents by virtue of their secretion of interferons (IFN-γ).  Immune Cytokines  Interferons

The cytokines produced by the two Th subsets perform cross-regulatory role. An activated Th2 cell secreting IL-4,-5,and 6 downregulates local Th1 cells in the neighborhood, whereas Th1 cytokines downregulate Th2 responses.

Tables  Fc receptors  Immune Cytokines  Immunoglobulins

[] diagram - helper T cells and phagocytic response to tumor cells [] diagram - HIV binding via CD4 receptors [] micrograph germinal center with helper T cells [] tem - helper T & B cell []

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. . . since 10/06/06