Abstract Extract The nature of the relationship between humoral immunity, characterised by the presence of serum antibodies, and delayed hypersensitivity, demonstrated by skin reactions, was a puzzle to early immunologists. The observation, made in 1945, that delayed hypersensitivity could be transferred from a sensitised to a normal animal by leucocytes but not by serum suggested that these two forms of immunity were separable. Soon after this observation was made a number of neonatal diseases associated with various immune deficiencies were described in children. The clinical features of these diseases and differences in their pathology also suggested a division between humoral and cellular immunity and, furthermore, indicated a role for the thymus in immunity. But it was the fortuitous discovery in 1956 that bursectomized chickens were unable to produce antibody but retained their cellular immunity that first established a definite relationship between a primary lymphoid organ and an immune function. A...