Lymphocytes and a variety of supporting cells make up the cells of the immune system. Three major types of lym phocytes are recognized: B cells, T cells, and NK cells. Supporting cells interact with lymphocytes and play important roles in the presentation of antigen to lymphocytes and the regulation of immune responses. These cells include monocytes, macrophages, neutrophils, basophils, eosinophils, reticular cells, dendritic cells, follicular dendritic cells, Langerhans’ cells, and epithelioreticular cells. In addition, a series of specialized epithelial and stromal cells provide the environment for many immune reactions to occur by secreting specific substances that regulate growth, migration, and the activation of effector and supporting cells.
Supporting cells in the lymphatic organs are organized into loose meshworks.
In lymph nodules, lymph nodes, and the spleen, reticular cells and the reticular fibers produced by these cells form elaborate meshworks. Lymphocytes, macrophages, dendritic cells, follicular dendritic cells, and other cells of the immune system reside in these meshworks and in the loose connective tissue of the body; Langerhans’ cells are found only in the middle layers of epidermis. At these sites, they carry out their mission of surveillance and defense. In the thymus, epithelioreticular cells form the structural meshwork within the tissue. Despite their name, these cells neither produce nor are related to reticular fibers.
Different types of cells in lymphatic tissue are identified by specific cluster of differentiation (CD) markers on their surface.
Different lymphatic and hematopoietic tissue cells possess unique cell surface molecules. These specific markers, called cluster of differentiation (CD) molecules, are designated by numbers according to an international system that relates them to antigens expressed at different stages of their differentiation. CD molecules can be visualized by immunohistochemical methods using monoclonal antibodies and are useful in identifying specific subtypes of lymphatic or hematopoietic cells. Some CD markers are expressed by cells throughout their entire life; others are expressed only during one phase of differentiation or during cell activation. Table 14.1 lists the most clinically useful markers.