Involution of the thymus is similar in its progression to that of eutherian species. The structure and cellular morphology of the spleen, lymph nodes and bone marrow are also similar to those of the placental mammals. A remarkable happening regarding involution of lymphoid organs occurs in the marsupial mice (Antechinus swainsonii and A. stuartii). Studies have shown that the thymus undergoes involution before the onset of puberty. In the males, the stress response to breeding leads to involution of the spleen and lymph nodes; the gut being unaffected. At the end of the breeding season, there is an abrupt and absolute mortality of the males in which it is apparent that the involution of organs associated with immunity plays a role. This 'die back' is unique to the species. Demonstration of cytomegalovirus and adenovirus particles in the tissues of many marsupial species in association with stressful situations suggests that there is a fine balance between health and compromized immunity.
There is evidence to support retardation in the initial phases of the humoral response. This is based on observations where the response to an inoculated bacteriophage by a rabbit produced more antibody, more rapidly than Didelpbis. Similar observations have been made in koalas (Phascolarctos spp.), where it is thought to be due to a delay in B cell activation because B cell numbers are present in similar proportion to those in placental mammals. In Monodelphis it has been shown that the primary immune response consists almost equally of IgM and IgG. However, in general, the humoral response in marsupials has the primary features that typify an immune response in placental mammals, including the ability to mount antibody-based or immediate hypersensitivity responses.
As in eutherian species, the initiation of the ability to mount a cellular response is dependent on the thymus, although there is evidence to support the possibility of differences in the effector mechanisms of marsupials. This has been reported in Didelphis and Setonix. In the latter, a study has been made of the delayed hypersensitivity response to 2,4-dinitrofluo-robenzene from both topical application and intradermal injection. It showed that a more prolonged regime was required to achieve a eutherian-like response following topical application and a longer latent period was observed with the intradermal administration. This reduced delayed hypersensitivity expression, particularly in contact hypersensitivity, has also been shown to occur in Trichosurus, where the use of oxazolone failed to induce a contact hypersensitivity. It has been found that the skin of Trichosurus has low numbers of Langerhans cells as compared to the guinea pig - the latter had 4-5 times the density. A lack of, or a reduced response to, lym-phokines has also been suggested.
The lymphokine responsiveness is of interest because of the many reports of mycobacterial infections in various marsupial species. While most of these infections have resulted in typical granulomatous reactions characteristic of tubercle bacilli, there have been exceptions. The most imporrant is the infection of Trichosurus with Mycobacteria bouis. This usually results in a disseminated systemic infection in which granuloma formation, a normal characteristic of the disease, is an infrequent finding. A similar finding is seen in Didelphis with Mycobacteria intracellular infection. It has been shown that Trichosurus has a cell-based response to mycobacterial antigens, but a study with a specific cytokine, macrophage chemotactic factor (MCF). showed that the Trichosurus macrophages did not respond to MCF. This was despite the fact that an MCF was produced by Trichosurus macrophages that was effective against macrophages of the guinea pig. This unusual finding requires further investigation, but it may explain why tuberculosis in the brush-tail possum is a disseminated disease.
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