Leukocyte

a type of cell formed in the myelopoietic, lymphoid, and reticular portions of the reticuloendothelial system in various parts of the body, and normally present in those sites and in the circulating blood (rarely in other tissues)

Description
Leukocytes can be isolated e.g. by centrifugation of whole blood and retrieved in the white (sometimes green) Buffy-coat interphase between the red pellet of erythrocytes and the translucent blood plasma. Under various abnormal conditions, the total numbers or proportions, or both, may be characteristically increased, decreased, or not altered, and they may be present in other tissues and organs. Leukocytes represent three lines of development from primitive elements: myeloid, lymphoid, and monocytic series.

Categorization
Different types of lymphocytes fulfill a variety of functions in inflammatory and immune-response reactions. They are functionally subcategorized into B cells (producing antibodies for immediate immune responses, with some serving as long-term memory cells), natural killer cells, which remove infected or cancer cells, and T cells. T cells are again subdivided into three types, helper T cells (CD4+), which coordinate immune responses and are important to anti-bacterial defense, cytotoxic T cells (CD8+), which remove virus-infected cells, and &#947;-&#948; T cells, which are intermediates between helper and cytotoxic T cells and natural killer cells. Monocytes take up pathogens (akin to neutrophils), but also serve the long-term display of pathogen fragments to T-cells for further defense responses. Monocytes can leave blood vessels and enter other tissues, where they are referred to as macrophages and mediate local immune responses. On the basis of features observed with various methods of staining with polychromatic dyes (e.g., Wright stain), a basic categorisation of leukocytes is that into granulocytes (neutrophils, basophils, and eosinophils; all containing lysosomes and surrounding small intruders) and agranulocytes (lymphocytes and monocytes). Cells of the myeloid series are frequently termed granular leukocytes, or granulocytes; cells of the lymphoid and monocytic series also have granules in the cytoplasm but, owing to their tiny, inconspicuous size and different properties (frequently not clearly visualized with routine methods), lymphocytes and monocytes are sometimes termed nongranular or agranular leukocytes. Granulocytes are commonly known as polymorphonuclear leukocytes (also polynuclear or multinuclear leukocytes), inasmuch as the mature nucleus is divided into two to five rounded or ovoid lobes that are connected with thin strands or small bands of chromatin; they consist of three distinct types: neutrophils, eosinophils, and basophils, named on the basis of the staining reactions of the cytoplasmic granules. Cells of the lymphocytic series occur as two, somewhat arbitrary, normal varieties: small and large lymphocytes; the former represent the ordinary forms and are conspicuously more numerous in the circulating blood and normal lymphoid tissue; the latter may be found in normal circulating blood, but are more easily observed in lymphoid tissue. The small lymphocytes have nuclei that are deeply or densely stained (the chromatin is coarse and bulky) and almost fill the cells, with only a slight rim of cytoplasm around the nuclei; the large lymphocytes have nuclei that are approximately the same size as, or only slightly larger than, those of the small forms, but there is a broader, easily visualized band of cytoplasm around the nuclei. Cells of the monocytic series are usually larger than the other leukocytes, and are characterized by a relatively abundant, slightly opaque, pale blue or blue-gray cytoplasm that contains myriad extremely fine reddish-blue granules. Monocytes are usually indented, reniform, or shaped similarly to a horseshoe, but are sometimes rounded or ovoid; their nuclei are usually large and centrally placed and, even when eccentrically located, are completely surrounded by at least a small band of cytoplasm. Syn white blood cell