Since their discovery in 1984, the function of the gamma delta subset of T cells has been the subject of much debate. The fact that large numbers of these cells reside in the epidermis and the intestinal epithelium suggests that they play an important role in the surveillance of epithelial barriers to invading organisms. Recent studies have shown that these cells are particularly prominent in the intestinal mucosa of young ruminants. In vitro studies have shown that this subset of cells selectively proliferate in response to antigens from both M. tuberculosis and M. bovis. These findings suggest that this subset of cells may play an important role in the pathogenesis of ruminant paratuberculosis.