Leukocytes, or white blood cells, are nucleated and are larger and less numerous (6000-10,000/Cl) than erythrocytes.
Leukocytes can be divided into 2 main groups, granulocytes and agranulocytes, according to their content of cytoplasmic granules. Each group can then be further divided on the basis of size, nuclear morphology, ratio of nuclear to cytoplasmic volume, and staining properties. Two classes of cytoplasmic granules occur in leukocytes: specific and azurophilic granules. Specific granules are found only in granulocytes; their staining properties (neutrophilic, eosinophilic, or basophilic) distinguish the 3 granulocyte types. Azurophilic granules occur in both agranulocytes and granulocytes.
Their content of lytic enzymes suggests that they function as lysosomes. Unlike the RBCs, all leukocytes can leave the capillaries by squeezing between endothelial cells, a process termed diapedesis, and enter the surrounding connective tissue in response to infection or inflamma tion. The types and levels of activity expressed by extravascular leukocytes depend upon the specific cell type