Alan WIlliams is noted for his seminal contributions to the field of leucocyte membrane glycoproteins (that is, differentiation antigens). He played a leading role in the development of approaches to the purification and structural analysis of cell surface antigens. His comprehensive characterization of the structure of the rat Thy-1 antigen, as well as the application of monoclonal antibodies to the designation and subsequent isolation of multiple new leucocyte antigens, were exemplary. His discovery that Thy-1 is structurally related to immunoglobulin led directly to the concept of the immunoglobulin (Ig) superfamily, which embraced a spectrum of cell surface molecules involved in a variety of cell recognition systems. He was a very strong advocate in support of the rat as a model animal in the study of immunological phenomena. He was energetic and courageous, as well as radiating enthusiasm for immunological research, inspiring others, critically analysing accepted dogmas and setting high standards. In short, he was a brilliant research scientist.