White Blood Cells

White blood cells (leucocytes) are colourless, nucleated, amoeboid cells that lack haemoglobin, and there are far fewer of them than red cells - roughly 6000 to 8000 per cubic millimetre in a healthy adult. They are grouped into two broad categories based on whether their cytoplasm contains visible granules. Granulocytes, differentiated in the bone marrow, include three types: neutrophils (also called heterophils or polymorphonuclear cells, with a nucleus of 3-4 lobes joined by thin threads) are the most abundant, at 60-65% of all WBCs, are strongly phagocytic, and swarm to infected tissue in large numbers; eosinophils, with a distinctly bilobed nucleus, make up only 2-3% of WBCs, are non-phagocytic, and rise sharply during parasitic infections and allergic reactions; and basophils are the rarest granulocyte, at just 0.5-1.0% of WBCs, with a large lobed (but not thread-joined) nucleus, and they secrete heparin, serotonin and histamine, driving inflammatory reactions. Agranulocytes, differentiated in the lymph glands and spleen, include two types: lymphocytes, at about 28% of WBCs, have a large round nucleus and little cytoplasm, and split into B cells (which produce antibodies to neutralise foreign substances) and T cells (which carry out cell-mediated immunity); and monocytes, at 1-3% of WBCs, are large phago …