ABSTRACT
Molecules, such as cytokines, chemokines, adhesion molecules, major histocompatability complexes (MHC), and antibodies, link the innate and adaptive arms of the immune system (Fig. 2). The innate immune system provides the first line of immune defense and is composed primarily of neutrophils, macrophages, and dendritic cells. Under non-stress conditions, these professional phagocytes gain rapid entry into infected tissues to clear pathogens by receptor-mediated phagocytosis, leading to the production of free radicals and the release of enzymes that kill the ingested microorganisms. The adaptive immune system is primarily composed of B and T lymphocytes, which require prior exposure to pathogens for immune activation. The B lymphocytes (B cells) produce and secrete antibodies, which are particularly effective in protecting animals against extracellular pathogens. The T lymphocytes are made up of several subpopulations. Helper T cells of the type I class (THI) participate in inflammatory, cytotoxic, and some antibody responses. Helper T cells of the type II class (THII) facilitate primarily antibody-mediated responses. Cytotoxic T cells (TC) and their innate counter-
parts, the natural killer (NK) cells, lyse and kill host cells infected with intracellular viruses and bacteria. Less welldefined gamma delta T cells (gd T cells) appear to have tissue-healing and immune-modulating roles that vary in significance across species.
