Page 222 - Veterinary Immunology, 10th Edition

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FIG. 8.2 The distinction among autocrine, paracrine, and
endocrine effects. Cytokines differ from hormones in that most of
their effects are autocrine or paracrine, whereas hormones usually
act on distant cells in an endocrine fashion.

When cytokines bind to receptors on target cells, they affect cell
behavior. They may induce the target cell to divide or differentiate,
or they may stimulate the production of new proteins.

Alternatively, they may inhibit these effects—preventing division,
differentiation, or new protein synthesis. Most cytokines act on
many target cell types, perhaps inducing different responses in each
one, a feature that is called pleiotropy. Conversely, many cytokines

may act on a single target, a feature known as redundancy. For
example, IL-3, IL-4, IL-5, and IL-6 all affect B cell function. Some
cytokines work best when paired with other cytokines in a process
called synergy. For example, the combination of IL-4 and IL-5

stimulates B cells to make immunoglobulin E (IgE) and hence
triggers an allergic response. Synergy can also occur in sequence
when, for example, one cytokine induces the target cell to express
the receptor for another cytokine. Finally, some cytokines have

opposing effects and may antagonize the effects of others. The best
example of this is the mutual antagonism of IL-4 and IFN-γ.

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