The study of immunology and the immune system in evolution. All vertebrates can recognize and respond to nonself-molecular configurations on microorganisms, cells, or organic molecules by utilizing a complex recognition system termed the immune response. The presence of lymphocytes and circulating antibodies has been documented in all extant vertebrate species. However, the existence of induced, specific reactions directly homologous to the immune repertoire of vertebrates has not been clearly established in invertebrates.
The role of phagocytic cells in engulfing foreign pathogens has been documented in virtually all metazoan organisms. Phagocytic cells possess a limited capacity to discriminate self from nonself, and this is due in part to the presence of lectins (molecules capable of binding specifically to various sugars) on their surface. Although there is no evidence to suggest that invertebrate lectins and vertebrate immunoglobulins are homologous structures, sufficient diversity exists within lectins of certain species to indicate that these types of molecules and their cellular expression on phagocytes might serve as a primitive and universal recognition mechanism. See Immunoglobulin, Lectins
All true vertebrates possess cells clearly recognizable as lymphocytes and can carry out T-cell functions, such as graft rejection, and show the capacity of B cells to synthesize and secrete immunoglobulins. True lymph nodes are not present in vertebrate species more primitive than mammals, but birds possess aggregates of lymphoid tissue probably serving a similar function. See Cellular immunology, Lymphatic system
Humans possess five major classes or isotypes of immunoglobulin: IgG, IgM, IgA, IgE, and IgD. The IgM molecule is the first immunoglobulin to appear in ontogeny, and the first to appear in phylogeny. Immunoglobulins of cyclostomes, elasmobranchs (sharks and rays), and many teleost fishes consist only of IgM polymers. Immunoglobulins possessing heavy chains distinct from the μ-like heavy chains of those groups are present in some lungfish (Dipnoi) and in anuran amphibians (frogs and toads). Dipnoi have a low-molecular-weight non-IgM immunoglobulin (termed IgN). Birds possess IgM and IgA immunoglobutins, but also possess a non-IgM immunoglobulin similar to that of amphibians as their major immunoglobulin class. This immunoglobulin has been termed IgY. IgG immunoglobulins containing gamma chains clearly homologous to those of the humans and of true mammals are found only within the three subclasses of living mammals, namely, eutherians, metatherians (marsupials), and monotremes (for example, the echidna). See Immunological ontogeny
Although the precise nature of the precursors of the specific elements of the immune system in evolution remains to be determined, the genetic and cellular events which lead to the capacity for specific immune recognition, diversification, and reactivity occurred early in vertebrate evolution. See Immunity, Immunology