Normal white blood cells (leucocytes) in peripheral blood are of five types: three of them contain granules and are termed granulocytes (neutrophils or polymorphs, eosinophils and basophils) and the other two types are monocytes and lymphocytes (see Chapter 4). Granulocyte and monocyte production occurs in the bone marrow and is controlled by growth factors (see Table 1.1). External stimuli (e.g. infection, fever, inflammation, allergy and trauma) act on stromal and other cells to liberate cytokines, e.g. interleukin 1 (IL-1) and tumour necrosis factor (TNF), which then stimulate increased production of these growth factors. The earliest recognizable granulocyte precursors are myeloblasts. These undergo a final division followed by further maturation into promyelocytes, myelocytes, metamyelocytes and, finally, mature granulocytes (neutrophils, eosinophils and basophils).
Function of white cells
The primary function of white cells is to protect the body against infection. They work closely with proteins of the immune response, immunoglobulins and complement. Neutrophils, eosinophils, basophils and monocytes are all phagocytes; they ingest and destroy pathogens and cell debris. Phagocytes are attracted to bacteria at the site of inflammation by chemotactic substances released from damaged tissues and by complement components. Opsonization is the coating of cells or foreign particles by immunoglobulin or complement; this aids phagocytosis (engulfment) because phagocytes have immunoglobulin Fc and complement C3b receptors (see below). Killing involves reduction of pH within the phagocytic vacuole, the release of granule contents and the production of antimicrobial oxidants and superoxides (the ‘respiratory burst’).
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