resistance manifested by a species (or by races, families, and individuals in a species) that has not been immunized (sensitized, allergized) by previous infection or vaccination; much of it results from body mechanisms that are poorly understood but differ from those responsible for the altered reactivity associated with the specific nature of acquired immunity; in general, innate immunity is nonspecific and is not stimulated by specific antigens.self. SYN: natural immunity, nonspecific immunity.