White Blood Cells and the Immune Response

Abstract

Blood contains about 7.5 × 10⁸ white cells (leucocytes) per dl, although this figure varies widely, even within a healthy individual. Unlike red cells and platelets, white cells are not confined to blood, but are present in the tissues also, where they fulfil many of their important functions; lymph nodes in particular contain large numbers of lymphocytes. There are two broad classes of white cell: the non-granular leucocytes, and the granulocytes, which contain densely staining granules in their cytoplasm (Table 6.1). Polymorphonuclear (PMN) granulocytes are distinguished by the appearance of their nuclei, which have multiple lobes, whereas monocytes have a round nucleus. PMN granulocytes are classified as neutrophils, eosinophils or basophils on the basis of the size, shape, number and staining characteristics of their granules. They may spend only a few hours in blood before entering the tissues.