Immune systems are generally characterized by their ability to distinguish between self and non-self cells, tissues, or molecules, and to eliminate the non-self (Janeway et al., 2005). While successful strategies for the detection and elimination of pathogens are present at all levels of animal evolution, the possibility of fusion or contamination with cells from one's own species still exists in many marine metazoans (e.g., Porifera, Cnidaria, tunicates) but has practically disappeared in vertebrates. The immune system in vertebrates is a complex and intricate system that can recognize non-self and provide protection from a wide variety of pathogens. While there is a high degree of interconnectivity between its components, the immune system can be loosely divided into two subsystems, the innate and the adaptive immune systems. Innate immunity is a non-specific, inducible response to pathogens. It is immediate in action, yet short-lived. The adaptive immune system, consisting of antibodies, B cells, T cells, and the major histocompatibility antigens, is much more specific, but takes longer to activate. It also features immunological memory and can augment itself to respond more quickly and with greater specificity to future infections of similar pathogens. Innate immunity is present in all phyla, whereas adaptive immunity is present only in jawed vertebrates.

There is a conspicuous paucity of evidence for a gradual transition from the innate immune system of invertebrates to the recombinatorial immune system of vertebrates. Astonishingly, the adaptive immune system appeared quite suddenly, around 450 million years (myr) ago with the emergence of the gnathostomes. Where the genes for the components of the adaptive immune system came from is a mystery. These genes seem to have somehow "jumped" into the genome of a jawed vertebrate about 450 myr ago (Janeway et al. 2005).

In this review we show that knowledge of immunity in lower vertebrate and invertebrate species is now increasing rapidly. Thus, elucidating the details of the origin of the immune systems from a comparative point of view in the vertebrates' closest relatives may finally lead to a rational explanation of immunology's "big bang" - and add to the understanding of our own immune system.

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