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Humoral response: The humoral immune system involves proteins called immunoglobulin antibodies that fight against disease and infection. These antibodies, which are secreted by white blood cells called B–cells, detect and bind to foreign substances that enter the body. Once the antibody detects a foreign substance in the body, the antibody attaches to it. This action signals other immune cells to destroy it. There are five classes of immunoglobulin antibodies: immunoglobulin A (IgA), immunoglobulin D (IgD), immunoglobulin E (IgE), immunoglobulin G (IgG), and immunoglobulin M (IgM). Each antibody is specific for a certain invader. IgA antibodies are primarily found in the nose, airway passages, digestive tract, ears, eyes, saliva, tears, and vagina. These antibodies protect body surfaces that are frequently exposed to foreign organisms and substances from outside of the body. The IgA antibodies make up about 10–15% of the antibodies found in the body. IgG antibodies are the smallest, but most abundant antibodies in the body, making up 75–80% of all of the antibodies in the body. They are present in all body fluids. In addition, they are the only antibodies that can cross the placenta during pregnancy. Therefore, the IgG antibodies of a pregnant woman help protect her fetus. IgG antibodies are considered to be the most important antibodies for fighting against bacterial and viral infections. IgM antibodies are the largest type of antibody. They are found in the bloodstream and lymph fluid. The IgM antibodies are the first antibodies that are produced in response to an infection. They also stimulate other immune system cells, including macrophages, to produce compounds that can destroy invading cells. IgM antibodies normally make up about 5–10% of all of the antibodies in the body. IgD antibodies are found in small quantities in the tissues that line the abdominal and chest cavity of the body. The function of IgD antibodies is not well understood. Researchers believe they play a role in allergic reactions to some substances, such as milk, medications, and poisons. IgD and IgE are present in very small amounts in normal human serum. IgE antibodies reside in the lungs, skin, and mucous membranes. They induce allergic reactions against foreign substances like pollen, fungus spores, parasites, and animal dander. IgE antibody levels are often high in people who have allergies. When IgE is active, the antibody triggers an allergic reaction called a hypersensitive reaction. An allergic response occurs when the immune system overreacts to substances called allergens, which are normally harmless in healthy individuals. The immunoglobulin antibodies detect and bind to the allergens. These antibodies also trigger the release of chemicals, including histamine, which cause allergic symptoms such as runny nose, sneezing, and hives. Cell–mediated immune response: The second part of the immune system, called cell–mediated immunity, involves white blood cells that help destroy harmful invaders that enter the body. This response helps prevent disease and infection. Cytotoxic T–cells destroy cells infected with viruses and cancers. When a cell is infected with a virus or it is cancerous, the molecules on the cell's surface are changed. Cytotoxic T–cells identify these specific molecules as harmful substances and destroy the infected cells. Cytotoxic T–cells contain pouches, called granules, which are filled with chemicals that kill infected cells on contact. These cells are also involved in organ transplant rejection. The cytotoxic T–cells attack the donated organ because it is perceived as an infected body cell. Natural killer (NK) T–cells are similar to cytotoxic T–cells because they recognize and destroy body cells that have become infected with viruses or cancer. They also have granules that are filled with chemicals that destroy infected cells on contact. Unlike cytotoxic cells, the NK T–cells do not need to recognize a specific molecule on the surface of other immune cells in order to become activated. Instead, they attack cells that do not have external molecules that label them as body cells. Macrophages, another type of white blood cell, are found inside the tissues of humans. Macrophages are phagocytes, which means they are able to engulf foreign substances that enter the body. Macrophages are in a resting state until chemicals that are released during an immune response activate them. Upon activation, these cells travel toward the site of injury and they engulf disease–causing organisms, called pathogens. Once a macrophage ingests a pathogen, the pathogen is trapped inside the cell's food vacuole. Enzymes and toxic substances inside the cell start to ingest the foreign substance. The cells then secrete chemicals called interferons, lysozyme, and other factors that stimulate other immune cells to respond to the foreign invaders and destroy them.
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