Human Immune System
The immune system is composed of cells and tissues that defend the body against pathogens. Initial responses are nonspecific because they do not target a particular pathogen. The skin (largest organ of the body) and mucous membranes are the body's first line of defense, serving as a physical barrier and providing chemical defenses. As pathogens enter the body, the immune system's inflammatory response initiates the release of histamines and prostaglandins. This increases blood circulation, thereby enabling white blood cells to migrate to the infection site.
Because many pathogens can exist only in a very narrow temperature range chemicals are released to increase body temperature, making it more difficult for pathogens to survive and reproduce. Interferon (a protein released by cells infected by viruses) causes surrounding cells to produce an enzyme that prevents viruses from making proteins and RNA. The most important nonspecific defense is mounted by three kinds of white blood cells: neutrophils, macrophages and natural killer cells. Each of these types of cells has a specific mechanism by which to attack and destroy pathogenic organisms.
If a pathogen is able to survive the body's nonspecific defenses, specific immune responses are triggered. Specific defenses include humoral immunity and cell-mediated immunity. Substances such as proteins, carbohydrates and lipids are found on the surface of viruses and bacteria. If these macromolecules trigger the immune response, they are called antigens.
Immunity against pathogens in the blood and lymph is called humoral immunity. B cells (lymphocytes) produce specific proteins, known as antibodies, which bind to specific antigens, tagging them for destruction by phagocytes.
In cell mediated immunity, killer T cells or cytotoxic T cells (lymphocytes), transfer proteins to a pathogenic cell, causing fluid to leak out of the membrane. The pathogenic cell ruptures and is destroyed. Both humoral and cell-mediated responses happen simultaneously and are regulated by helper T cells.
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This work was supported by National Space Biomedical Research Institute through NASA cooperative agreement NCC 9-58.