Relationship between Thymus-dependent Cells and Humoral Immunity

Abstract

RECENT discoveries^1–7 seem to indicate that humoral immune responses to some antigens require the interaction of thymus-dependent and thymus-independent cells. An antigen sensitive cell of thymic origin would exist, which is responsible for recognizing antigen, and which differs from the antigen insensitive cells from bone marrow capable of synthesizing antibody. Support for this view comes from experiments involving transplantation of thymus or thoracic duct lymphoid cells into animals which were rendered incapable of forming antibody to sheep cells (SRBC) by irradiation or neonatal thymectomy. An interaction between thymus cells and precursors of antibody forming cells was first demonstrated by injecting irradiated mice with SRBC mixed with thymus cells, marrow cells, or both¹. The antibody response in mice given both cell types exceeded the sum of responses to the individual cell types. Thymus or thoracic duct lymphocytes mixed with SRBC and injected into neonatally thymectomized mice elicited antibody responses comparable with those in normal mice^2–5. From these experiments and others in which adult mice were thymectomized and irradiated but protected from death by the presence of bone marrow, and injected with SRBC mixed with thoracic duct lymphocytes, it seems clear that antibody synthesizing cells were derived from thymus-independent cells by way of precursors in the marrow. Thus responses to SRBC, like those to other antigens, rely on a thymus-independent population for antibody production. We conclude that the lymphoid system is organized into thymus-dependent and thymus-independent lymphoid populations and that the latter are involved in synthesizing and secreting antibody.