infectious diseases; but the two cell types may differ in that MHC molecules may not play a critical role in antigen presentation to most y T cells.

subunits. The an3 T-cell receptors rec-ognize short peptides derived from protein antigens,embedded in the groove of antigen-presenting mol-ecules encoded by genes of the major histocompati-bility complex (MHC). In their roles as helper (CD4+)and cytotoxic (CD8+) T cells, ac T cells constitute themajor component of T-cell immunity to infectiousagents and other antigens. Several years ago, a distinctsubset of T cells was discovered that express an antigenreceptor composed of variable y and 8 subunits.Although a great deal has been learnt of the diversity,development and subtype heterogeneity of y8 T cells[1], an understanding of their physiological roles in theimmune response and their antigenic targets has beenelusive. Recent progress in both of these areas holdsout the promise that significant advances may soon beforthcoming.The recent production of mice deficient in expressionof cat and/or y5 T-cell receptors, by means of targetedmutations that inactivate their genes for and/or 8T-cell receptor chains, allows for the first time directanalysis of the specific roles played by the differenttypes of T cell in immunity to infectious diseases.Obviously, if '6 T cells have a unique role in immunityto specific infectious diseases, T-cell receptor 8 chainmutant mice, which lack 68 T cells but have apparentlynormal c T cells, should be highly sensitive to suchdiseases. Examples of this sort may emerge with time,but none has yet been reported. Some infectiousagents, on the other hand, may be attacked by botha3 and y6 T cells. In this case, mice mutant for thegenes encoding both