The immunity raised in vertebrates by the use of the classic adaptive immune mechanisms involves the major histocompatibility complex, B- and T lymphocytes associated with RAG1 and RAG2, and somatic hypermutations. The adaptive immune system applying set-aside cells enabling the host organism to respond faster and more specifically to a second encounter with a pathogen is developed to the highest degree in mammals. The social amoebae Dictyostelium discoideum form a primitive multicellular organism but with some specialization between compartments. The cnidarians comprise—among others—jellyfish and sea anemones and are equipped with different types of immune-related cell types, which are able to communicate with vertebrate-like cytokines. The cellular equipment of crustaceans allows representatives of this group to sense and phagocytose foreign elements and potential pathogens, leading to protection against reexposure to pathogen. The cellular machinery of earthworms has been extensively studied, and several cell types described.