White Blood Cells
(Microsoft Encarta)
COMPOSITION OF BLOODAbout 55 percent of the blood is composed of a liquid known as
plasma. The rest of the blood is made of three major types of
cells: red blood cells (also known as erythrocytes), white blood cells
(leukocytes), and platelets (thrombocytes).White Blood CellsWhite blood cells only make up about 1 percent of blood, but their small number
belies their immense importance. They play a vital role in the body?s immune
system?the primary defense mechanism against invading bacteria, viruses, fungi,
and parasites. They often accomplish this goal through direct attack, which
usually involves identifying the invading organism as foreign, attaching to it,
and then destroying it. This process is referred to as
phagocytosis.White blood cells also produce antibodies, which are released into the
circulating blood to target and attach to foreign organisms. After attachment,
the antibody may neutralize the organism, or it may elicit help from other
immune system cells to destroy the foreign substance. There are several
varieties of white blood cells, including neutrophils, monocytes, and
lymphocytes, all of which interact with one another and with plasma proteins and
other cell types to form the complex and highly effective immune system.Platelets and ClottingThe smallest cells in the blood are the platelets, which are designed for a
single purpose?to begin the process of coagulation, or forming a clot, whenever
a blood vessel is broken. As soon as an artery or vein is injured, the platelets
in the area of the injury begin to clump together and stick to the edges of the
cut. They also release messengers into the blood that perform a variety of
functions: constricting the blood vessels to reduce bleeding, attracting more
platelets to the area to enlarge the platelet plug, and initiating the work of
plasma-based clotting factors, such as fibrinogen. Through a complex mechanism
involving many steps and many clotting factors, the plasma protein fibrinogen is
transformed into long, sticky threads of fibrin. Together, the platelets and the
fibrin create an intertwined meshwork that forms a stable clot. This
self-sealing aspect of the blood is crucial to survival.
Resumos Relacionados
- Plasma
- Blood Type
- Splenomegaly
- H.i.v Transmission
- Cancer Vaccines
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