Lymphocytes are a type of agranulocytes, which are in turn a type of leucocytes (white blood cells).
Agranulocytes (including lymphocytes) are distinguished from the other category of leucocytes - which are called granulocytes, because agranulocytes do not contain chemical-filled cytoplasmic vesicles called "granules".
There are several different types of lymphocytes, such as B-Cells, T-Cells, and Natural Killer Cells.
20-25% of all white blood cells are lymphocytes.
Small lymphocytes are 6-9 um in diameter while large lymphocytes are 10-14 um in diameter. They have round or slightly indented nuclei and cytoplasm forming a sky-blue coloured rim around the nucleus of each cell.
Lymphocytes are extremely important for the body's immune system because they mediate immune responses, including antigen-antibody reactions. Each of the different types of lymphocyte plays a different role in immune processes:
- B-Cells develop into plasma cells - which secrete antibodies.
- T-Cells attack invading viruses, cancer cells, and transplanted tissue cells.
- Natural Killer Cells attack a wide range of infectious microbes and some tumour cells - specifically, spontaneously arising tumour cells.
For more information about other components (sometimes called the 'constituents') of blood, see the page about the structure and functions of blood. This may interest students of holistic massage, reflexology, beauty therapies, or health-related courses.