"Not proper mammals": immunity in monotremes and marsupials.

Immune systems and responses in Monotremata and Marsupialia are reviewed. The Monotremata (Prototheria) are egg-laying mammals. Few studies have been carried out on monotremes. The structure of the lymph nodules of Tachyglossus aculeatus is unusual, and the occurrence of IgG in this species is noteworthy: IgG has not yet been found in any non-mammal. A number of Marsupialia (Metatheria) species have been used as immunological models. Generally immune responses are somewhat slower and less accentuated than in placental (eutherian) mammals. Of interest is the presence of cervical and thoracic thymuses in several marsupials. Marsupials are born very immature and possess rather rudimentary immune responses at birth: the neonate may provide a helpful model for immune ontogenesis. Marsupials have a full repertoire of immunoglobulin classes. MHC Class II (but not Class I) gene polymorphism may be limited. Studies using molecular biology techniques are awaited to elucidate the structural organization of the immune components and to determine similarities and differences between marsupials' and other animals' immune systems.