The white blood cells, or leukocytes, are unpigmented, nucleated cells which are the essence of the immune system. They are ubiquitous as they are found in the bloodstream, lymphatic vessels and nodes, spleen, and thymus. Through the arteries, arterioles, and capillaries, they reach every tissue of the body to protect us against foreign pathological microorganisms, viruses, parasite larvae, and cancer cells.
There are three types of white blood cells: granulocytes, lymphocytes, and monocytes. The granulocytes, in turn, are composed of three specialized leukocytes: neutrophils, basophils, and eosinophils. Lymphocytes consists of T-lymphocytes and B-lymphocytes (or simply T and B cells). Monocytes, on the other hand, are a mononuclear phagocytic cell (destroys pathogenic microorganism through phagocytosis). All of them originate from stem cells in the red bone marrow and mature in the spleen and thymus.
The granulocytes are so called because of the granule-like structures found in their cytosol. The neutrophils are polymorphonuclear leukocytes, which are highly specialized to attack and destroy bacteria through phagocytosis, using hydrolitic enzymes. Eosinophils are granulocytes whose cytoplasm contain rounded granules; they react before the presence of parasites and their parasitic larvae. Basophils, like eosinophils, play an important role in parasitic infections and allergies, appearing in many inflammatory reactions that cause allergic symptoms; they also produce histamine and heparine.
Lymphocytes are spherical in shape and contain an oval nucleus. B-lymphocytes produce antibodies by transforming into plasma cells when the detect the presence of an antigen (virus, protozoan, bacteria); these plasma cells mark the antigen for the cytotoxic T cells and monocytes can identify and destroy it. T-lymphocytes (cytotoxic T-cells) react when they detect a pathogen (virus, bacterium, etc) that has been marked by a B-lymphocyte by destroying it. There are other types of T-lymphocytes, such as helper T-cells and natural killer cells.
Below, schematic picture of the different types of white blood cells.
