Evolution of the Immune System

The human immune system is a complex assembly of innate (i.e., germline encoded) and adaptive (i.e., somatically generated) mechanisms. Among Metazoa, invertebrates use mostly innate mechanisms, whereas vertebrates have maximized the usage of somatic mechanisms, allowing each individual to adapt to the changing pathogenic environment. Historically, Metazoa have developed immune systems from a limited number of shared elements, hence the homologies observed between phyla (e.g., in the conservation of some signaling cascades). The different immune systems of Metazoa offer also examples of analogies that reflect the intensity of the selection pressures. Analogies in diversifying immunoreceptors are the most striking.