White blood cells
White blood cells (leukocytes) are the body's mobile warriors in the battle against infection and invasion. There are three types of white blood cell: granulocytes, lymphocytes, and monocytes. There are, in turn, three kinds of granulocyte: neutrophils, eosinophils, and basophils. (Granulocytes are called that because they contain granules that hold digestive enzymes.) Neutrophils kill invading bacteria by ingesting and then digesting them. Eosinophils kill parasites, and are involved in allergic reactions. Basophils also function in allergic reactions, but are not well understood.
Lymphocytes are key parts of the body's immune system. There are two kinds of lymphocyte: T cells and B lymphocytes. T cells direct the activity of the immune system. B lymphocytes produce antibodies, which destroy foreign bodies. Monocytes, the largest kind of white blood cells, enter the tissues of the body and turn into even larger cells called macrophages. These eat foreign bacteria and destroy damaged, old, and dead cells of the body itself.
