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Abstract
Extract
Infectious diseases and neoplasia are major threats to an animal's survival. While acquired immunity is essential in resistance to infectious disease its value in providing protection against tumours is debatable. Infectious organisms can be divided arbitrarily into two types: those that tend to live extracellularly and those that live intracellularly. This distinction is appropriate, in an immunological sense, because, with few exceptions, immunity to extracellular organisms, and their products, is mediated by antibodies whereas immunity to intracellular organisms is largely a function of cell mediated immunity (CMI). CMI, in this context, can be defined as antibody-dependent, T-lymphocyte mediated immunity. This definition is important because other forms of “cell mediated” immunity occur. These are either antibody-in-dependent and mediated by cells bearing receptors for the Fc regions of reacted antibodies or else they are antibody-independent and mediated by so-called natural killer cells.