Although they constitute only about 5% of circulating T cells and have apparently limited variability, the so-called gd T cells are turning out to have an important role in the front lines of the body9s defenses against invading pathogens. Early on, for instance, researchers found that they are much more concentrated in mucosal surfaces, where most pathogens enter the body, than in the blood. More recent studies of the mechanism of action of gd T cells suggest that despite their limited antigen-recognition capabilities they are nonetheless able to deal with the broad spectrum of antigens they are likely to encounter at mucosal surfaces. This is partly because they recognize antigens that may appear on epithelial cells that are under stress, say, because they are infected. In addition, gd T cells may help the body damp down autoimmune reactions and repair tissues damaged by inflammation.