Parasites differ enormously in their abilities to evade normal host defenses. Intracellular protozoa are capable of surviving and multiplying within phagocytes and other cells whereas most nematodes do not multiply within the host. Both T lymphocyte-mediated immune responses and antibody production are important in limiting parasite invasion and in eliminating them. Of the nonspecific factors, macrophages, natural killer cells, and nonantibody serum factors can cause damage to parasites. Malnutrition impairs immunity, most notably cell-mediated processes. The number of T lymphocytes is reduced and there are significant alterations in T cell subsets. Mucosal antibody response is blunted, complement activity is decreased, and microbicidal capacity of phagocytes is reduced. Such changes in host resistance are important determinants of the final outcome of host-parasite interactions.