(WBCs) (Gk. leukos-white, kytos-cell)
Leucocytes are colourless, nucleated, amoeboid and phagocytic cells. Due to their amoeboid movement they can squeeze out of blood capillaries. This is called diapedesis.
The size is about 8 to 15 μm. There are about 5000 to 9000 WBCs per cu mm of blood. This is called total WBCs count. The average life span is about 3 to 4 days.
Formation of WBCs is called leucopoiesis. It occurs in red bone marrow, spleen, lymph nodes, tonsils, thymus and Payer's patches. Increase in number of WBCs is called leucocytosis whereas decreases in their number is called leucopenia. Leukemia is a pathological increase in number of WBCs and is commonly called blood cancer. The dead WBCs are destroyed by phagocytosis in blood, liver and lymph nodes.
Leucocytes are of two types - granulocytes and agranulocytes.
Granulocytes : Granulocytes show granular cytoplasm and lobed nucleus. They are of three parts: neutrophils, eosinophils and basophils.
Neutrophils : In neutrophils the cytoplasmic granules are stained with neutral dyes. The nucleus is three to five lobed hence are called polymorphonuclear leucocytes or polymorphs. They constitute about 70% of total WBCs. They are phagocytic in function and engulf microorganisms.
Eosinophils (Acidophils) : Eosinophils (Acidophils) show the cytoplasmic granules which are stained with acidic dyes such as eosin. The nucleus is bilobed. They constitute about 3% of total WBCs. They are non-phagocytic and their number increases during allergic conditions. They show anti-histamine property. Increase in number of eosinophils is called eosinophilia.
Basophils : Basophils have the cytoplasmic granules that are stained with basic dyes such as methylene blur. The nucleus is twisted. They constitute about 0.5% of total of total WBCs. They are non-phagocytic. They release heparin (anticoagulant) and histamine (involved in inflammatory and allergic reaction).
Agranulocytes : Agranulocytes show absence of granules in the cytoplasm and nucleus is not lobed. They are of two types - lymphocytes and monocytes.
Lymphocytes : Lymphocytes show large round nucleus. They constitute about 30% of total WBCs. They produce antibodies and responsible for immune response of the body.
Monocytes : Monocytes are the largest of all WBCs and show large kidney-shaped nucleus. They constitute about 5% of total WBCs. They are phagocytic in function. At the site of infection monocytes enlarge and differentiate into macrophages which engulf microorganisms and remove cell debris. Hence they also called scavengers.