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Humans
Humans have two basic kinds of immunity that allow our bodies to distinguish between self and non-self and thus guard against threatening microbes, parasites or substances. Innate immunity is genetically inherited and responds fairly similarly to all antigens, or things that elicit immune system responses. This type of immunity is carried out by the skin, mucus membranes, stomach acids and other bodily fluids, including enzymes secreted in the bloodstream. It is also accomplished through specialized cells, such as phagocytes, macrophages and eosinophils that directly seek out and attack foreign bodies. Lymphocytes, or white blood cells, mediate acquired immunity. While part of it is genetically based, acquired immunity is an ongoing process of cellular learning that enables human bodies to remember every antigen they have ever encountered and mount quicker, more effective immune responses upon re-exposure. T-lymphocytes originate in the thymus gland and directly attack foreign bodies with antigens they specifically recognize. B-lymphocytes, from bone marrow, are responsible for secreting immunoglobulins, antibodies specific to foreign antigens. The acquired immune system recognizes the Major Histocompatibility Complex (MHC), the antigens in the body's own cells; otherwise, autoimmune disease results.
Other Mammals
Aside from species and individual variations in the makeup of the MHC, humans and nonhuman mammals have basically similar immune systems. The greatest immunological difference among mammals lies between marsupials, such as opossums and kangaroos, and eutherians, like humans and gazelles. Marsupials emerge from the germ-free uterus and are carried in germ-exposed pouches, where their acquired immune systems first develop. Eutherians, however, emerge from the uterus with highly developed adaptive immunity.
Other Vertebrates
Non-mammalian vertebrates, or back-boned animals, also have highly sophisticated acquired and innate immune systems. Although less well-characterized, avian, or bird, immune systems are essentially the same as mammalian ones. Even less is known about reptilian, amphibian and fish immune systems. However, MHC antigens have been discovered in amphibians. While they do have other lymphoid tissues, reptiles lack lymph nodes, structures that in other vertebrates trap foreign bodies and store immune compounds.
Invertebrates
The least researched animal immune systems are those of invertebrates, animals without backbones, such as insects, crabs and starfish. Invertebrates generally rely upon innate immunity and lack acquired immunity. All multi-cellular organisms have some kind of innate immunity. Aside from these generalizations, as the immunologist E.S. Loker asserted in a 2004 scientific review paper, invertebrate immune systems are "not homogenous, not simple, not well-understood."