Granulocytes in health and disease

White cells (leucocytes) mediate inflammatory and immune responses and are key to the defence of the host against microbial pathogens. Subpopulations of leucocytes include granulocytes—neutrophils, eosinophils, and basophils; monocytes; and lymphocytes. Neutrophils comprise half the peripheral circulating leucocytes and are characterized by heterogeneous primary and secondary granules and a segmented nucleus. Maturation from the haematopoietic stem cell occurs in the bone marrow and takes 10 to 14 days. Neutrophilia—defined as an increase in the circulating neutrophil count to greater than 7.5 × 106/µl, usually occurs as an acquired reactive response to underlying disease. Causes include infection, particularly bacterial; drugs; malignancies, and hereditary conditions. Neutropenia—defined as a reduction in the absolute neutrophil count to less than 1.5 × 106/µl, is of particular importance because, when severe (<0.5 × 106/µl), it markedly increases the risk of life-threatening infection. Causes include drugs and toxins, postinfectious, nutritional deficiencies, autoimmune, large granular lymphocytosis, and congenital. Disorders of neutrophil function include chronic granulomatous disease, leucocyte adhesion deficiency, myeloperoxidase deficiency, and Chediak–Higashi syndrome. Monocytes share a common myeloid precursor with granulocytes, present antigens to T cells, produce several important cytokines with immunomodulatory and inflammatory functions, and are the precursors to resident tissue macrophages. They are especially important in defence against intracellular pathogens. Causes of monocytosis (>0.9 × 106/µl) include chronic infection, autoimmune diseases, and malignancy. Basophils are nonphagocytic granulocytes that function in immediate-type hypersensitivity. Basophilia (> 0.2 × 106/µl) is seen in myeloproliferative disorders, hypersensitivity reactions, and with some viral infections.