ABSTRACT Lymphocytes, the cells competent to initiate immune responses, can be divided into two major groups: thymus-derived or T cells responsible for "cellular immunity" (e. g. delayed hypersensitivity reactions) and bursa (or bursa-equivalent) derived or B cells which produce immunoglobulin (antibody) molecules and are involved in "humoral immunity". "Accessory" cells, such as monocytes (or macrophages), polymorphonuclear leucocytes and mast cells act in an auxiliary manner by facilitating antigen processing or presentation, or by liberating factors which modify the various manifestations of the immune response. A variety of interactions between T and B cells and between lymphocytes and accessory cells have been described in both cellular and humoral immunity. Antigen-activated T cells produce factors with various activities: some are involved in recruiting inflammatory cells, others activate macrophages and enhance their microbicidal activities, and others modify B-cell responsiveness either by facilitating or suppressing it. These factors are instrumental in T-cell regulation of immune responsiveness. Antibody produced by B cells also plays a role in immunoregulation, acting either as an immunopotentiating influence or as a negative feedback, e. g. when complexed with antigen, turning off the response of T or B cells. A detailed knowledge of the precise manner in which cells involved in immunity are regulated is essential for an understanding of how the foetus, an essentially "foreign transplant", can survive to term in its immunologically mature "alien" host.