Even unicellular organisms have many mechanisms for protection, such as restriction endonucleases, antimicrobial peptides, and RNA interference. In multicellular organisms, specialized immune cells have evolved, such as macrophages, dendritic cells, natural killer lymphocites, T and B lymphocytes (in terms of effector cells and memory cells). Such cells are capable of phagocytosis, recognition, and killing of foreign cells as well as removing their own cells that have been alteres by damage, infection, cancer or senescence. Additional humoral factors, such as the complement cascade and natural IgM antibodies, have developed that cooperate with cellular immunity to fight infection and maintain homeostasis. Defensive mechanisms composed of germline-encoded receptors constitute one particular system, known today as innate immunity.
In jaw vertebrates, this system is supplemented with a second system, the adaptive immune response, which, in contrast to the innate immune response, is based on diversification of immune receptors and immunologic memory in each individual. During evolution, each newly evolved defense mechanism typically did not replace the previous one, but instead, formed to supplement it. This resulted in a layered structure of the immune system which developed into a highly sophisticated adaptive immune response. As a result there is no one single immune system, but rather a sophisticated network of various defensive mechanisms operating on different levels, ranging from mechanisms common for every sessile cell in the body to specialized mobile immune cells and responses in the whole mechanism.
The innate immune system is first in the line of host defence. As molecular research developed, the evolutionary origin of the molecules involved in innate immune mechanisms was revealed to be ancient..."