Studies of immunoevolution have revealed diverse strategies within the animal kingdom. The immune system evolved to prevent the threat of extinction by microbial pathogens. Phagocytosis is the most ancient response and at other phylogenetic levels the complexity of immune responses reflects the level of species development. Whether vertebrate immunocytes evolved from invertebrate leukocytes is still questionable without firm information concerning the structural and functional properties of their receptors. Antibodies appear exclusively in vertebrates, however serological and biochemical evidence reveals members of the tg superfamily among certain invertebrates. Cytotoxic cell immunity with limited T cell receptor‐major histocompatibility complex (TCR‐MHC) repertoire could have developed at an early stage of invertebrate evolution. Specificity and anamnesis arc considered hallmarks of induced immune responses and there is evidence for both with respect to cellular and humoral responses in invertebrates. Cytokines, which are released by a variety of activated immune cells, exert profound effects on cells of the immune system and the host defense system in general. They seemed to have evolved early. Future research must emphasize a search for pattern recognition receptors and those receptors which are determined by rearranging gene families.