Lymphocytes: Guardians of the Adaptive Immune System
Lymphocytes are a type of white blood cell that are essential for the adaptive immune response. They are agranulocytes, meaning they lack the specific granules characteristic of granulocytes like neutrophils, eosinophils, and basophils. Lymphocytes are produced in the bone marrow, but they mature and differentiate in different lymphoid organs. They circulate throughout the body in the blood, lymph, and connective tissues, continuously monitoring for signs of infection or abnormal cells.
Key Features of Lymphocytes
- Round or oval nucleus: Lymphocytes typically have a large, spherical nucleus that occupies most of the cell volume. The nucleus is surrounded by a thin rim of cytoplasm.
- Scant cytoplasm: The cytoplasm of lymphocytes is relatively sparse and contains few organelles.
- Size: Lymphocytes are typically smaller than granulocytes, with a diameter ranging from 6 to 15 μm. However, activated lymphocytes or natural killer (NK) cells can be larger, up to 18 μm in diameter.
- Lifespan: Lymphocytes have a wide range of lifespans, from hours to many years, depending on their specific function.
- Surface receptors: Lymphocytes express a variety of surface receptors, including the T cell receptor (TCR) on T lymphocytes and immunoglobulin (antibody) molecules on B lymphocytes. These receptors enable lymphocytes to recognise and bind to specific antigens.