Maturation of the immune system and immunodeficiency.

I will concentrate on recent work that has improved our understanding of the earliest phases of lymphocyte maturation, since this work provides a basis for the analysis of various forms of immunodeficiency. However, the reasons for interest in primary lymphocyte differentiation extend beyond a consideration of immunodeficiency. According to the clonal selection theory, which has received considerable support from experimental work, the broad range of foreign antigens which may enter the body are recognised by a repertoire of lymphocytes, each specific for a particular antigen. There is evidence that this repertoire of lymphocytes is acquired during the primary differentiation of lymphocytes that occurs in ontogeny. In addition, although the mechanisms of tolerance to 'self' antigens are poorly understood studies on lymphocyte ontogeny may provide important clues. Lymphocytes, like other blood cells, belong to a renewal system and production of new lymphocytes is not confined to ontogeny. Hence, in discussing the primary differentiation of lymphocytes, sites of lymphocyte production in the adult must also be considered.