Lymphocytes are the central cells of the immune system. Lymphocytes responsible in adaptive immunity which contribute in diversify, memory, specificity and self and non self recognition. White blood cells also plays an important role in phagocytosis, presenting antigens and secreting cytokines.
Lymphocytes constitute of 20% to 40% of white blood cells. Lymphocytes circulate in the blood and lymph. They are capable to migrate into the tissue spaces and lymphoid organs to have higher degree of protection. Lymphocytes are divided to T lymphocytes, B lymphocytes and natural killer cells.
B lymphocytes name derives from the letter designation from its site of maturation, in the bursa of Fabricius in birds; the name turned out to be apt, for bone marrow is its major site of maturation. B lymphocytes can be differentiated from other lymphocytes by the synthesis and display of membrane bound antibody at the membrane surface of B lymphocyte.
T lymphocytes name derives from their site of maturation in the thymus. T lymphocytes have T cell receptor at the membrane surface. T lymphocytes are not able to recognize free antigen. They only recognize antigen when it is bound to MHC molecules at the antigen presenting cell.
Natural Killer Cell
Natural killer cells or NK cells are large and granular. They display cytotoxic activity against tumour by antibody-dependent cell-mediated cytotocixity (ADCC).
The mononuclear phagocytic system consists of monocyte (circulate in blood) and macrophage (in the tissues). Monocytes circulate in the bloodstream. They enlarge and then enter tissues. They differentiate into specific macrophage or into dendritic cells. Macrophages serve different functions in different tissues and named according:
- Alveolar macrophage in lung
- Histiocytes in connective tissue
- Kupffer cells in liver
- Mesangial cells in kidney
- Osteoclasts in bone
- Microglial cells in brain
Macrophages exhibit phagocytic activity, secrete various cytotoxic proteins, express class II MHC molecules where allow them to act as antigen presenting cells.
Granulocytes are classified into neutrophil, eosinophil and basophil according the morphology of the cell and cytoplasmic staining properties.
Neutrophils, also called polymorphonuclear leukocyte (PMN) are mutilobed nucleus. The cytoplasm stains with both acid and basic dyes. Neutrophils constitute 50%70% of white blood cells.
Eosinophiles are bilobed nucleus and a granulated cytoplasm that stains with the acid dye eosin red. Eosinophile constitute 1%3% of white blood cells.
Basophils, less than 1% of white blood cells are one lobed nucleus and heavily granulated cytoplasm that stains with the basic dye methylene blue. Basophils play role in allergy response by releasing of pharmacological active substances from their cytoplasmic granules. Both he neutrophil and eosinophil are phagocytic cells. But not basophil.