During the last decade rapid progress has been made in the area of microbial immu nity. New
conceptual frameworks have emerged with regard to the processing and presen tation of peptides
and nonpeptide antigens from microbes especially bacteria to T cells in the context of
classical MHC class I and nonconventional MHC I molecules. Experimen tal models have been
instrumental in defining some of these pathways for generating pro tective immune responses
against microbes which form the basis for the design of new vaccines. New evidence has
stressed the importance of innate immunity in microbial in fections. The concept of dichotomy
within T helper cells has revealed the role of these cell types in resistance and
susceptibility to microbial-mediated pathology. These latest devel opments in microbial
immunity are discussed in this volume. Natural killer cell development is known to be regulated
by the presence of MHC class I antigens. Receptors for MHC class I molecules on NK cells have
been discovered. Interaction of these receptors with their specific ligands leads to inhibition
of cytotoxicity. Vinay Kumar and colleagues review NK cell differentiation and ontogeny and
functions of NK cells in experimental animals. Lewis Lanier discusses the role of a newly
discov ered molecule DAP 12 in KIR and other receptor-mediated signal transduction in NK
cells. Eric Long describes the regulation of immune response by inhibitory receptors.
