NK cell receptors: emerging roles in host defense against infectious agents.

Natural killer (NK) cells have the ability to become activated under the appropriate conditions by utilizing one or more cell surface receptors that are capable of inducing NK cell cytokine production and/or cytotoxicity. The expression of a variable array of inhibitory receptors on the surface of NK cells acts to counterbalance the positive signals initiated through activating receptors. Increasing evidence suggests an important role for both activating and inhibitory NK cell receptors in an appropriate and controlled NK response to infectious agents.