White cells live in the buffy coat between reds and plasma
Leukocytes—white blood cells—defend against infection and foreign matter, generally larger than red cells, in three main groups: granulocytes, lymphocytes, and monocytes. All arise from bone-marrow hematopoietic stem cells and keep nuclei, unlike red cells and platelets.
White cells are scarce: roughly 1 percent of a healthy adult's blood volume, against 40 to 45 percent for red cells, yet immunity depends on them. Spun in a centrifuge, blood separates into red cells at the bottom, plasma above, and a thin pale band of nucleated cells between them called the buffy coat. That band can turn greenish when neutrophils are abundant, because they carry a haem-containing enzyme, myeloperoxidase. The scientific term leukocyte simply joins Greek roots for white and cell.
Five main kinds are recognised, and students remember their usual order of abundance with the phrase Never Let Monkeys Eat Bananas: neutrophils, lymphocytes, monocytes, eosinophils, basophils. Granulocytes have lobed nuclei and visible granules, while agranulocytes have round nuclei. Neutrophils carry three to five nuclear lobes linked by thin strands, which makes them look as if they had several nuclei.
Each type has a speciality. Eosinophils, whose granules stain pink-orange with eosin, are few in blood but plentiful in the linings of the airways, gut and lower urinary tract; they release chemicals that kill parasites such as hookworms and tapeworms, too big for any single cell to swallow, and their numbers swing with time of day, season and menstruation. Basophils, under half a percent of the total and hard to study, release histamine to widen and loosen blood vessels and heparin to slow clotting, and they call other white cells to the site.
Lymphocytes live mostly in the lymphatic system. B cells produce antibodies that tag and block invaders. CD4 helper T cells coordinate the response with signalling molecules and serve as the key gauge of immune health in HIV infection, while CD8 cytotoxic T cells kill virus-infected and tumour cells flagged by MHC class I molecules.
Source: White blood cell
