The Experimental Pathology Unit's main research interest was lymphocytes, particularly focusing on the role of the thymus in the immunological competencies of lymphocytes. The Unit compared populations in normal and thymectomised mice, examined proliferation and differentiation in response to antigen, and looked at precursor cells. Within four years of its existence, the Unit (Miller and Mitchell) demonstrated that thymus-derived lymphocytes or T-cells did not give rise to the cells that formed antibodies and that this instead was the function of bone marrow-derived lymphocytes, the B-cells.

In 1969 the Unit's research aims were targeted primarily at the function of thymus-derived cells and thymus-marrow interactions. Later, interests expanded to include studies of in vitro antibody response, immunological memory and the function of activated T-lymphocytes. One particular project in the early 1970s was the nature of the T-cell antigen receptor. They subsequently disproved ideas of a T-immunoglobulin comparable to the immunoglobulin receptor on B-cells. This lead into an increasing focus on the process of T-lymphocyte function and activation in response to antigen, and on the T-cell subset responsible for delayed type hypersensitivity (DTH) reactions. The Unit demonstrated that in DTH the triggering antigen must be associated with certain products of the major histocompatibility complex (MHC) on the surface of macrophages. Between 1978 and 1981 the Unit began work on developing immortalised antigen-specific T-cell lines, invaluable in facilitating biochemical studies of T-cell differentiation, activation and function.