Immunity may be defined as a resistant state conferred on a living organism by a wide variety of cells and molecules that tend to eliminate pathogens or aggressive agents, thus ensuring the individual’s integrity. Immunologists often make a distinction between natural, or innate, and adaptive immunity, although the latter is also quite "natural" when understood as a basic physiological phenomenon. It might be advisable to use instead the concept of nonspecific versus specific, but even this is not entirely satisfactory, because the limits are not so clear. This is well illustrated by the opposed views that were expressed at the end of the nineteenth century by those, like Metchnikoff, who supported the cellular basis for immunity, centered on the phagocytic properties of the macrophage and those, like Ehrlich, who gave a major role to circulating antibodies, thus supporting humoral immunity. In fact, both mechanisms are part of the organism defenses; and the macrophage, which was initially considered to have nonspecific phagocytic potentialities, turned out to be able to acquire some specificity by opsonisation—that is, the binding of antibodies of the various isotypes to its Fc receptors. Much more recently, the ability of macrophages to process and present protein antigens proved definitively that all these partners could be linked and that a clear-cut distinction was primarily a scholastic dispute. Defense mechanisms are quite diversified in the living world, although vertebrates have developed a highly complex immune system endowed with an adaptive response that can face a huge potential antigenic repertoire because of a sophisticated genetic organization that may express at any given time millions of different and specific immunoglobulins or T cell receptors (TCRs) produced by B cells and T cells, respectively. Invertebrates have also developed immune defenses against pathogens that involve, in addition to phagocytic cells or soluble enzymes, mechanisms like those described in insects, which have a limited repertoire of genetically encoded peptides endowed with diverse bacteriolytic, antiviral, or antiparasite activities.
One may consider that the defense mechanisms that operate in vertebrates are organized on successive levels. The first one involves cutaneous and mucosal barriers that ensure a physical protection, as well as a chemical one, including enzymes such as lysozyme that can have a bactericidal effect. The second level is used whenever pathogens have entered the bloodstream. Examples of efficient but nonspecific mechanisms that can be mobilized quickly are (a) phagocytosis by monocytes and macrophages, (b) inflammatory reaction that produces many cytolytic factors, (c) lytic activity initiated by components of the complement cascade, (d) natural killer (NK) cells, and (e) various cytokines, with special reference to interferons that block viral replication, and so on. Simultaneously, the adaptive immune response is initiated, leading to the occurrence of the first wave of IgM antibodies, which are soon replaced by more efficient IgG, and/or to the emergence of cytotoxic T lymphocytes, which are of prime importance in destroying virus-infected cells. Vaccination will reinforce immunity by stimulating a specific immune response to major dangerous pathogens, allowing the organism to react quickly with the appropriate B- and T-cell responses when a natural infection takes place.
