THE MAMMALIAN LYMPHOCYTE

Abstract

SUMMARY Thoracic duct lymphocytes are for the most part newly formed cells; there is only minor circulation of lymphocytes between blood and lymph. Quantitative studies of thoracic duct lymphocyte production show that, in the cat and rabbit, the number of lymphocytes daily entering the blood via the thoracic duct is sufficient to replace those already in the blood 3‐0‐5‐0 times daily. Occasionally the blood lymphocytes rise during an experiment in which thoracic duct lymph is being collected. This may mean that in these cases a very considerable number of lymphocytes enters the blood from the lymphoid tissues directly. Mitotic counts both in sections and in isolated nuclei indicate that lymphocyte formation proceeds most actively in the thymus, and measurements of the nucleic acid turn‐over of lymphoid tissues confirm this. There is no direct correlation between the total mass of the lymphoid tissues and the level of the blood lymphocytes. Extirpation of 90% of the lymphoid tissues is usually followed by a transient fall in the level of the blood lymphocytes, and sometimes results in the new formation of lymphoid tissue in liver or lungs. Individual lymph nodes which have been removed do not regenerate. Lymphoid tissue seems to be essential to the body. Lymphocytes can disappear rapidly from the blood even in the absence of stomach and intestines. During the first 5 days after removing about 40% of the bone marrow, the blood lymphocytes rise to nearly three times their pre‐operative level. The subcutaneous injection of egg albumin produces a local accumulation of lymphocytes which in the course of 6 hr. becomes transformed into macrophages. Tissue cultures of thoracic duct lymphocytes have yielded conflicting results concerning the developmental potentialities of the lymphocyte. The peculiar nature of lymphocyte movement has been used in an attempt to establish the transformation of lymphocytes into macrophages. Observation of lymphocytes in transparent chambers in rabbit's ears has shown that they can remain alive for as long as 26 days, though they were never observed to undergo transformation into other cells. Chemical and spectrographic studies indicate that small lymphocytes have a very low growth rate. There is a close relationship between the suprarenal cortex and the lymphoid tissues, upon which some of the cortical hormones exert a depressant effect. The adrenotropic hormone of the pituitary exerts a similar action upon lymphoid tissue by stimulating the secretion of the suprarenal cortex. The precise effect of cortical hormones upon lymphocytes is still not clear; some of the degenerative changes which have been described can be observed also in normal lymphoid tissue. Cortical hormones have no action upon lymphocytes in vitro. After subcutaneous injection of antigen, antibodies appear in progressively increasing amounts in the regional lymph nodes and in their efferent lymph, reaching a maximum after 6 days. At the same time the lymph nodes undergo hyperplasia with increase in the number of lymphocytes in their efferent lymph. Tissues and exudates rich in macrophages have a low antibody content. It has been suggested that suprarenal cortical extract is a potent stimulus to antibody formation, but evidence has also been adduced to the contrary. The plasma cell may play a major part in antibody formation, though not in the formation of plasma proteins apart from antibody. The study of the precise cells concerned in antibody formation has revived the controversy concerning the origin of the plasma cell, whether from a lymphocyte precursor or from a specific plasmoblast. Lymphocytes constitute about 10% of the nucleated cells of normal adult human marrow, though in childhood they may be considerably more numerous. The absolute count of lymphocytes in rabbit bone marrow was estimated at 61,000 per cu.mm., and in the guinea‐pig 165,000 per cu.mm. The lymphocyte content of bone marrow is subject to marked variation; this may in part reflect difficulties in cell identification. Lymphoid nodules are found in adult human red marrow in one case out of three. They do not show the usual mitotic activity characteristic of lymphoid nodules, and are probably focal accumulations of haematogenous lymphocytes. Viable lymphocytes, marked with acriflavine and injected into the blood, were subsequently found in the bone marrow. Lymphocytes are found in considerable numbers in the mucous membrane and submucous tissues of the alimentary canal. Experiments to determine whether or not these lymphocytes are excreted into the lumen of the alimentary canal are unsatisfactory. No quantitative data are available on the number of lymphocytes present in the connective tissues of the body.