Positive and negative regulation of leukocyte activation by protein tyrosine phosphatases.

Many of the initial signal transduction events essential for eliciting an immune response are controlled by the concerted action of protein tyrosine kinases and protein tyrosine phosphatases. However, positive or negative regulation of the cellular response does not fall exclusively into the domain of either class of enzymes. Rather, distinct enzymes can be either positive or negative regulators of a molecular reaction and in certain circumstances, it is conceivable that a single enzyme may have both functions. The biological functions of two protein tyrosine phosphatases, SHP and CD45, serve to illustrate the dichotomy by which this class of enzymes regulate immune responses.