Gut-associated immune tissue, as represented by lymphocytes and lymphoid cell aggregates, occurs in all jawed vertebrates. Considering the lymphopoietic function of the cryptopatches in the mouse intestine, the lymphoid aggregates in the gastrointestinal lamina propria of birds and cold-blooded vertebrates seem to serve the same function, thus providing an explanation for their conservation during a period of over 400 million years. Some gammadelta+ intraepithelial T cells are likely to develop and mature in gut-associated immune tissues. Intraepithelial gammadelta+ T cells may therefore have represented the first step in the evolution of adaptive immunity, reinforcing the gastrointestinal defense against microbial invasion as a result of increasing traumatization by injury and infection due to jaw development in host fish.
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