of LRR-based DNA segments. There is no evidence for the recombination-activating genes (RAGs) that rearrange Ig and TCR gene segments in jawed vertebrates. The evolution of the AIS, based on Ig, TCR, and MHC molecules, is thought to have arisen from two major evolutionary events: the transfer of the RAG transposon (possibly of viral origin) and two whole genome duplications. Though the molecules of the AIS are well-conserved, they are also rapidly evolving. Yet, a comparative approach finds that many features are quite uniform across taxa. All the major features of the AIS arose early and quickly. Jawless fishes have a different AIS that relies on gene rearrangement to generate diverse immune receptors with a functional dichotomy that parallels Ig and TCR molecules. The innate immune system, which has an important role in AIS activation, is the most important defense system of invertebrates and plants. Types of acquired immunity Immunity can be acquired either actively or passively. Immunity is acquired actively when a person is exposed to foreign substances and the immune system responds. Passive immunity is when antibodies are transferred from one host to another. Both actively acquired and passively acquired immunity can be obtained by natural or artificial means. Naturally Acquired Active Immunity – when a person is naturally exposed to antigens, becomes ill, then recovers. Naturally Acquired Passive Immunity – involves a natural transfer of antibodies from a mother to her infant. The antibodies cross the woman's placenta to the fetus. Antibodies can also be transferred through breast milk with the secretions of colostrum. Artificially Acquired Active Immunity – is done by vaccination (introducing dead or weakened antigen to the host's cell). Artificially Acquired Passive Immunity – This involves the introduction of antibodies rather than antigens to the human body. These antibodies are from an animal or person who is already immune to the disease. See also Affinity maturation Allelic exclusion Anergy Immune response Immune tolerance Immunosuppression Original antigenic sin Somatic hypermutation Polyclonal response Notes and references Notes References Immune system