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The following HTML text is provided to enhance online readability. Many aspects of typography translate only awkwardly to HTML. Please use the page image as the authoritative form to ensure accuracy. the identification of nonforeign body cells. Two classes of MHC molecules exist, each containing two polymorphic polypeptide chains that traverse the cell membranes of body cells. Molecular transcription rates of both MHC classes are influenced by cytokines, thus providing an important amplification mechanism for T-cell responses. Both classes were originally recognized for their role in triggering rejection of transplanted tissues, but their larger roles in forming complexes with diverse kinds of foreign protein antigens is now known. Class I MHC molecules are expressed on virtually all nucleated body cells. MHC molecules of both types possess peptide-binding, Ig-like, transmembrane, and cytoplasmic regions. The peptide-binding regions of both types are the principal determinants of the specificities and affinities of peptide antigens that can be bound, wherever the Ig-like regions appear to be important for noncovalent interactions between the two molecular chains. Antigens associated with Class I MHC molecules are recognized by CD8+ T-suppressor lymphocytes. Class II MHC molecules are expressed primarily on cells involved in the presentation of foreign antigens (i.e., lymphocytes, macrophages, dendritic cells, endothelial cells, etc.). In contrast to Class I molecules, Class II molecules show differences in cell expression and cytokine responsiveness among these cell types. Class II-associated molecules are recognized by CD4+ T-helper cells. Memory. The exquisite recall exhibited by the immune system that enables it to mount a more vigorous and effective response whenever it is restimulated by a specific foreign antigen. Memory cells. Lymphocytes that have previously responded to a specific antigenic stimulus. They survive for exceedingly long periods and can respond rapidly to the same antigen. Monocyte. A large white blood cell with a single nucleus, with phagocytic, cytokine-producing, and antigen-processing capabilities. Tissue forms are called macrophages. Monokine. A cytokine secreted by a monocyte or macrophage. N Neopterin. A small protein produced by monocyte–macrophages (often in combination with IL-6 and other proinflammatory cytokines) that has immunosuppressive properties. Neutrophil. The most numerous of the white blood cells (also called polymorphonuclear leukocytes). The principal cellular participant in
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