Histological and immunohistological investigation of alimentary tract lymphoid tissue in the koala (Phascolarctos cinereus), brushtail possum (Trichosurus vulpecula) and ringtail possum (Pseudocheirus peregrinus).

The histological appearance and distribution of T cells, B cells and plasma cells were investigated for oropharyngeal tonsils, small intestinal lymphoid aggregations (Peyer's patches), caecocolic lymphoid patches and mesenteric lymph nodes of koalas (Phascolarctos cinereus), common brushtail possums (Trichosurus vulpecula) and common ringtail possums (Pseudocheirus peregrinus). The histological organisation and distribution of lymphoid cell subpopulations of these tissues were similar to those described in eutherian mammals, although some differences were found in comparison with previous descriptions of American opossum tissues. The main variation among the 3 species was in the structural organisations of the oropharyngeal tonsil and the caecocolic lymphoid patch which were more complex in the koala than in possums. In the koala the extensive crypts of the oropharyngeal tonsils and folding of the mucosa of the caecocolic lymphoid patch increased their surface area and, in addition, both structures had areas of epithelium heavily infiltrated with T and B cells. These features could indicate that these structures are important in immunological surveillance of orally presented antigens in koalas.