Physiology Topics
Leukocytes (white blood cells) recognize and destroy microbes and toxins, and release antibodies and chemical signals. They are classified as granular or agranular, and the granular leukocites are named according to how they stain. All leukocites have major histocompatibility (MHC) antigens protruding from their plasma membrane into the extracellular space, unique to each person (except identical twins).
Neutrophils are granular leukocytes that stain with a combination of basic and acid agents, and are the most abudant and mobile. Their granules are lysosomes, containing enzymes that damage tissue on release. They also secrete defensins, proteins that make hole on microbial plasma membranes, and can engulf bacteria and proteins.
Basophils are granular and stain with basic agents. Their granules contain histamine, heparin and seratonin, which intensify the inflamatory response.
Eosinophils are granular, stain with basic agents and are less mobile than neutrophils. Their granules contain peroxidases, oxidases, phosphatases and other enzymes that detoxify histamine and other substances released during allergic reactions. They also attack parasitic infections which are too large to phagocytize, and engulff antigen-antibody complexes.
Monocytes and lymphocytes are agranular leukocytes. Monocytes enlarge and diferentiate into macrophages as thety reach the target tissue. They engulf and break down bacteria and prioteins. Some macrophages reside in a particular tissue while others roam around and gather at sites as needed.
Lymphocytes are subdivided into T-cels, B-cells and natural killer cells (NKC). T-cells mature in the thymus, while B-cells mature in various places. B-cells destroy bacteria and inactivate toxins by producing antibodies. T-cells act directly on viruses, fungy, transplants, cancer cells as some bacteria. NKCs attack a wide variety of microbes and tumo cells.
Leukocytes leave the blood stream in response to signal from patogens or other leukocytes. The interact with the endothelium by rolling along it. Adhesion molecules called selectins on the plasma membrane of endothelial cells interact with leukocytes according to their type by identifiying glycoproteins o the leukocyte plasma membrane. Integrins on the leukocyte lasma membrane adhere to the endothlial cell and assist movement therough the blood vessel wall as the leukocyte seem to fit into junctions between endothelial cells. Some theories suggest that leukocytes are rather engulfed by endothelial cells on the inner side of the vessel and extruded on the outer side. (may add more info on this latter).
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