The role of interleukins in the modulatory action of T lymphocytes

Helper cells become aware of the presence of an antigen when it is exposed in the binding cleft of an MHC-II protein.  Whether the antigen is displayed by a B cell or a presentation cell, only a helper cell specific for the antigenic determinant and the MHC-II molecule holding it can bind and become activated.  An activated helper can advance the campaign against the antigen and its source in several ways.  First it installs receptors for a chemical signal molecule, interleukin, in its own membrane; then it begins secreting interleukin.  The binding of interleukin to its own receptors causes the helper cell to proliferate (see figure).  The secreted interleukin also induces multiplication of any activated cytotoxic T cells nearby that have recently encountered their specific antigens; more often than not, the helper cell and its cytotoxic neighbor will be responding to the same pathogen.

When bound to stimulated B lymphocytes, helper T cells produce an interleukin that encourages the B cells to secrete antibodies.  Yet another kind of interleukin energizes nearby macrophages.

Modulatory action of T lymphocytes.  (A) Helper cells regulate the humoral immune response by binding to B lymohocytes that display a unique antigen.  (B) The bound helper  secretes various interleukins, one of which causes the helper to multiply; another form of interleukin induces the B cell to secrete antibodies, which bind he antigens on the invading cell (A). (C)  In the cell-mediated responsce (which normally would be active simultaneously only if the invading microorganisms can infect host cells), a third kind of interleukin induces nearby bound cytotoxic T lymphocytes to kill their targets.  (D) A fourth interleukin activates nearby macrophages to ingest antibody-marked targets.

From Gould and Keeton, Biological Sciences, 6th edition

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