Leukocytes, or white blood cells, play an important role in the immune system. They are produced in the bone marrow and released into the blood when they become mature. There are several types of leukocytes, with functions ranging from protection against infection by microorganisms to protection against tumors and parasites. Leukocytes also play a major role in inflammation and allergic reactions.
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Neutrophils are the most abundant type of leukocyte. They are highly mobile cells and can move quickly through the body. Neutrophils are often the first responders in cases of injury or infection and are involved in early inflammation. Neutrophils are able to fight off bacterial and fungal infections by ingesting the invading organisms, which are killed and eliminated from the body.
Eosinophils combat infections, especially those from small parasites. They are also important in fighting off viral infections. Many viruses contain ribonucleic acid, or RNA, as their genetic material. Eosinophils contain molecules that destroy RNA, thereby destroying many viruses. Eosinophils are also involved in allergic responses, as well as the inflammatory process that causes asthma.
Basophils play a major role in inflammation, especially allergic reactions. They store histamine, a substance that dilates blood vessels and allows blood to flow into the tissues, causing inflammation. Histamine is released from the basophils in response to exposure to certain substances in the environment that cause allergic reactions, such as pollen. Basophils are not as mobile in the body as other types of leukocytes and are, therefore, involved in later phases of inflammation.
Lymphocytes are part of the adaptive immune system, which is able to recognize foreign matter and destroy it quickly. Lymphocytes include natural killer cells, which are able to recognize and destroy altered cells, such as cancerous or infected cells. T cells and B cells are also types of lymphocytes. These cells are involved in producing antibodies against invading germs, so they can be easily recognized and attacked if they invade the body again.
Monocytes are important in the innate immune system, the part of the immune system that removes infections without requiring any memory of the attacking organism. Monocytes protect tissues from invading foreign substances and can rapidly move through the body to sites of infection. Once they reach an invading organism or substance, they divide into cells called macrophages, which then ingest and destroy the organism. Often, small pieces of the destroyed organism are left behind for the lymphocytes to come into contact with and begin to build antibodies against, should the organism try to invade again.