Lymphocytes

Lymphocytes, including B and T lymphocytes, initially develop in the bone marrow and then undergo further maturation in lymphoid organs. Immature T cells or thymocytes travel from the bone marrow to the thymus where they acquire central tolerance and differentiate into subpopulations, such as CD4+ helper T cells and CD8+ cytotoxic T cells, with diverse functions. B and T cells are activated in response to specific antigens in secondary lymphoid organs and function as the major effector cells of the adaptive immune response. Following antigen exposure, B cells differentiate into plasma cells that make a single type of antibody that is specific for that antigen. The identification of lymphocytes within tissues and organs and their functions will be considered in more detail in the Lymphoid System session.

Lymphocytes range in size from ~9-18 microns in diameter and are generally described as small, medium, and large lymphocytes (see image below). B and T cells cannot be distinguished in routinely prepared blood smears; this requires the use of immunocytology to detect cell surface markers that are specific for the different B and T cell types.

• Identify lymphocytes of various sizes in this peripheral blood smear slide. Lymphocytes are abundant, representing ~20-40% of the circulating WBC populations, and are relatively easy to find in a smear. They typically have large, round basophilic nuclei with a thin rim of pale staining, basophilic cytoplasm.
• To review the ultrastructural features of lymphocytes in TEM images, please see Figure 12-11 in Junqueira’s Histology 14e.

Next, let’s look at monocytes and platelets.