The importance of thymus-dependent cells, or T cells, in the generation of a successful immune response was first realized in the early sixties. In the follow ing two decades, a succession of elegant experiments established the antigen specificity of T cells and their ability to perform both as regulatory and effector cells. T cells were shown to be essential in most immune reactions, playing a crucial role in augmenting the activity of effector T and B cells against 'foreign' antigen, as well as in the suppression of effector activity against self antigens. The means by which T cells...