Postgraduate Medical Journal (1987) 63, 931-935 Postgrad Med J: first published as 10.1136/pgmj.63.745.931 on 1 November 1987. Downloaded from Review Article Lymphocyte subpopulations R.J. Powell and J.S. Jenkins Department of Immunology, University Hospital, Queen's Medical Centre, Nottingham NG720H, UK. Invertebrates possess an innate non-specific immune number of lymphocytes that can react with a specific system that utilizes phagocytic cells to dispose of antigen and hence there is a maximization of the antigen. However, higher animals including man have number of lymphocytes that come into contact with developed an adaptive immune system which is the antigen.' specific in its reaction for a given antigen. Both The majority of lymphoid tissues within the spleen humoral (antibody-mediated) and cell-mediated reside in the white pulp which forms the sheath around immune responses are dependent on lymphocytes the splenic arterioles known as the periarteriolar which have the capacity to recognize and respond to lymphatic sheath (PALS). T cells are located around antigen. Ninety five percent of peripheral blood the central splenic arterioles in the paracortex while lymphocytes (PBL) are small lymphocytes comprising the B cells are found in the cortex in either primary or both T and B lymphocytes which are derived from secondary follicles. The PALS is surrounded by an Protected by copyright. pluripotential stem cells. 'lthough T and B cells are important marginal zone which contains the slowly morphologically identical, the two groups differ in recirculating B cell population. In the medulla there their cell surface structures and response to antigen. are both T and B cells and also plasma cells. The remaining 5% of PBL are the so called third Non-encapsulated lymphoid tissue is found in the population of lymphocytes which include large gran- gut, urogenital and respiratory mucosae and although ular lymphocytes (LGL). the T cells are localized in discrete areas, the B lymphocytes can be either organized into follicles or q distributed diffusely. Lymphocyte traffic T cells mature immlnologically during passage B lymphocytes through the thymus whiie B cells acquire maturity and immunological competence in the fetal liver and adult B cells develop in fetal liver but at about 8 weeks of bone marrow. Mature B cells migrate via the blood gestation the bone marrow becomes the major site of http://pmj.bmj.com/ and lymphatic circulation to the secondary lymphoid production. The earliest recognizable B cell is the large organs, spleen, lymph nodes and the mucosal pre-B cell with its characteristic cytoplasmic /t chain associated lymphoid tissue (MALT) of the gut, res- and these develop into B cells which exhibit surface piratory and genito-urinary systems. Within these IgM. The mature B cell can also express surface organs the lymphocytes tend to be segregated into immunoglobulin ofother classes, e.g. IgA or IgG, and predominantly T or B cell areas, and here they can, in these B cells can differentiate into plasma cells follow- close co-operation, recognize and respond to antigen. ing exposure to antigen. During the immune response The lymphocy'es recirculate non-randomly between some memory B cells are generated which enable the on September 24, 2021 by guest. the major lymphoid organs via the blood and lym- more rapid and aggressive secondary humoral phatic channels; the blood-borne lymphocytes enter immune response on further antigen challenge. These the lymph nodes between the high cuboidal cells ofthe memory B cells are located in the germinal centres of post capillary venules and leave via the efferent lymphoid organs. lymphatics. Following antigen entry into a draining Although B and T cells are morphologically similar, lymph node there is a transient retention of lym- the characteristic feature of B lymphocytes is the phocytes in the node for approximately 24 hours. This surface expression of immunoglobulin which can be mechanism is important because there is only a limited readily detected by immunofluorescence, as well as a number of other surface proteins including receptors Correspondence: R. J. Powell M.B., B.S., M.R.C.P. for the Fc portion of IgG (FcR), Class II major Received: 27 May 1987 histocompatibility complex (MHC) antigens and the © The Fellowship of Postgraduate Medicine, 1987 932 R.J. POWELL & J.S. JENKINS Postgrad Med J: first published as 10.1136/pgmj.63.745.931 on 1 November 1987. Downloaded from complement receptors denoted CR 1-4. The presence proteins, namely the class I antigens (HLA A, B and C) of CR2 on the cell surface is currently thought to be and the class II antigens (HLA DP, DQ and DR). The essential for the entry of the Epstein-Barr virus.2 All cellular distribution of class I and class II antigens these surface receptors enable the cell to respond to a differs enormously in that class I antigens are expres- variety ofstimuli and in particular the surface immuno- sed on the surface of all nucleated cells whereas the globulin acts as the antigen receptor on B cells. The distribution of class II antigens is very much more specificity of the antigen receptor and the subsequen- restricted, being expressed on B lymphocytes, macro- tly secreted immunoglobulin is maintained during B phages, monocytes, activated T cells and some endoth- cell differentiation into plasma cells. The mor- elial cells.3 Th cells recognize antigen only in associa- phologically distinct plasma cell does not possess tion with HLA DR whilst cytotoxic cells recognize surface bound immunoglobulin but its function is to antigen in association with class I antigens. secrete large quantities ofthe antigen specific immuno- The T helper population is characterized by the globulin. OKT4 (CD4) antigen. Activation of T helper cells is shown schematically in Figure 1 and the antigen presenting cell (APC) (macrophages and dendritic T lymphocytes cells) present antigen and MHC class II in concert. The Th cell, on recognizing the antigen, is induced to enter Pre-T cells migrate from the marrow to the thymus the early G1 phase. This GI Th cell has interleukin 1 and in that micro-environment they diversify and (IL-1) receptors and also secretes -y-interferon which differentiate into a variety of T cell subpopulations. then activates the APC. This activated APC produces The acquisition of a number of cell surface antigens IL-1 to further stimulate the IL-1 receptor bearing T accompanies these changes and interestingly many of helper cells which then move into the late GI phase them are transient. The vast majority of cells within and themselves secrete interleukin 2 (IL-2), which the thymus die before full maturation and only causes proliferation of the antigen-specific T helperProtected by copyright. approximately 1% of the cells entering the thymus and cytotoxic T cells, ultimately allowing full T cell leave as mature immunocompetent cells. Many of the activation to occur. deleted pre-T cells are presumably auto-reactive. Cytotoxic T cells (Tc) reside in OKT8 (CD8) T cells can be characterized by a fortuitous reaction population and recognize and kill target cells indepen- with sheep red blood cells (SRBC) whereby their dently of antibody by direct contact, probably by combination in vitro causes red cells to form a rosette inducing an alteration in cellular permeability leading around the T cell. The advent of monoclonal to osmotic lysis. Human Tc cells will lyse virally antibodies having specificities for T cell surface struc- infected cells by recognizing the viral antigens in close tures has enabled characterization of the repertoire of association with class I antigens on the cell surface. T cell receptors. The SRBC receptor has been defined The advantage of this dual recognition is that the as the OKT11 (CD2) glycoprotein and the OKT3 cytotoxic T cells are unable to recognize free viral (CD3) determinant defines a mature T cell population antigen, but will recognize and lyse virally infected which comprises the majority of total circulating cells thereby combating the infection at its source. Th lymphocytes. The OKT4 (CD4) determinant is cells are essential in Tc generation; they recognize the associated with a subset ofT cells which help or induce same antigen in association with class II glycoproteins http://pmj.bmj.com/ other lymphocytes, e.g., B cells and some T cells, and on an antigen-presenting cell surface and cause clonal are hence called T helper cells (Th). The OKT8 (CD8) expansion of Tc by releasing IL-2. positive subset of cells was classically thought to be involved with cytotoxic and suppressor functions, but the allocation of suppressor function to only T8 B and T cell activation bearing cells is currently debated. The transferrin receptor (CD9) is recognized by the monoclonal B lymphocytes recognize the surface configuration of on September 24, 2021 by guest. OKT9. an antigen rather than a particular polypeptide T lymphocytes function by 'dual recognition', that sequence. This recognition occurs via the B cell surface is, that they will only recognize foreign antigen when it immunoglobulin receptor and they also require a is associated with cell surface glycoproteins coded for second signal which is provided by a Tb cell which, as by genes within the major histocompatibility complex described above, recognizes antigen and the class II (MHC). The antigen-specific T cell receptor comprises antigen on the APC. Lymphokines secreted by a disulphide linked dimer of alpha and beta subunits. activated Th cells help to stimulate B cells to differen- Each receptor is unique and is associated with the tiate further into plasma cells or to become memory B OKT3 (CD3) glycoprotein. In man the MHC is cells. situated on the short arm of chromosome 6 and this gene complex encodes two groups of cell surface LYMPHOCYTE SUBPOPULATIONS 933 Postgrad Med J: first published as 10.1136/pgmj.63.745.931 on 1 November 1987.