The immune response is a defensive reaction against harmful substances, such as viruses or bacteria, which enter the body. The immune response helps protect the body from disease, infection, and cancer.
The immune system is a complex network of cells, proteins, tissues, and organs in the body that work together to fight off harmful substances and disease-causing microorganisms, called pathogens.
The immune system is also responsible for allergic reactions. An allergic reaction occurs when the body overreacts to substances called allergens, which are normally harmless in healthy individuals. For instance, if the immune system mistakes substances, such as pollen, fungus spores, parasites, or animal dander, for a harmful invader, the body will launch an attack. This reaction causes allergy symptoms such as sneezing, runny nose, and hives.
If the immune system is not functioning properly, individuals are more susceptible to diseases, infections, and cancers. This is because the immune system is the body's first line of defense against these conditions. Some patients may be born with immune disorders, such as Nijmegen breakage syndrome. Others may acquire immune disorders, such as HIV, later in life.
Some individuals may develop autoimmune system disorders, which cause the immune system to be overactive. Autoimmune disorders occur when the immune system attacks the body's cells because they are mistaken for harmful invaders, such as bacteria. These disorders can destroy body tissues, cause abnormal organ growth, and/or impair organ function.
The immune response can be broken down into two major parts:
the humoral immune response and cell-mediated response. The humoral immune system involves proteins called immunoglobulin antibodies that help signal immune cells to destroy harmful substances that enter the body. The second part of the immune system, called cell-mediated immunity, involves white blood cells that destroy foreign invaders in the body. There are several different types of white blood cells that have important roles in the immune response, including B-cells and T-cells.
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immune system organs
Bone marrow: Bone marrow is the soft, spongy tissue found inside bones. All of the cells of the immune system initially come from the bone marrow during a process called hematopoiesis. During this process, stem cells in the bone marrow either develop into mature immune cells or immature immune cells. The immature immune cells leave the bone marrow and mature in other areas of the body, such as the thymus gland.
Lymph nodes: The lymph nodes are small glands found throughout the body. The lymph nodes filter a type of bodily fluid called lymph fluid. The immune cells inside the lymph nodes capture foreign substances that are present in this fluid, preventing them from causing an infection.
Spleen: The spleen, which is a dark red oval organ on the left side of the body, filters the blood. The immune cells in this organ destroy foreign invaders that are present in a person's blood.
Thymus gland: The thymus gland, which is located in the front of the chest area, produces white blood cells called T-cells that help the body fight against disease and infection. This gland is most prominent early in life. Once an individual reaches puberty, the gland gradually decreases in size throughout adulthood.
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.
types of immunity
Innate immunity: All humans are born with innate (natural) immunity. Innate immune responses are both immediate and nonspecific. In other words, immune cells involved in innate immunity are not specific to just one type of foreign substance. Instead, the immune cells engulf substances that are identified as foreign. This response primarily occurs on the external barriers of the body, including the skin, nose, throat, and gastrointestinal tract. It is the body's first line of defense to prevent disease-causing organisms from entering the body.
Adaptive immunity: Adaptive immunity is a type of protection that develops over the course of an individual's life. Adaptive immunity involves the development of immunoglobulin antibodies that respond to specific foreign substances that enter the body. When individuals are exposed to certain foreign invaders, the body develops antibodies against the pathogens. Then, if the same substance enters the body in the future, the body is now prepared to respond quickly because the antibodies are already developed.
Passive immunity describes the immune system of babies who are less than six months old. Because a fetus' immune system is not fully developed, pregnant mothers pass immunoglobulin antibodies from their bloodstream, through the placenta, and to the fetus. These antibodies are an essential part of the immune system. They identify and bind to harmful substances that enter the body. When this happens, other immune cells are triggered to destroy the foreign substance. As a result, antibodies help prevent disease and infection. This provides the fetus with passive immunity.
Immunoglobulin G (IgG) is the only antibody that crosses the placenta to the fetus during pregnancy. IgG antibodies are the smallest, but most abundant antibodies, making up for 75-80% of all the antibodies in the body. They are present in all body fluids, and they are considered to be the most important antibodies for fighting against bacterial and viral infections.
These antibodies help protect the fetus from developing an infection inside the womb.
Immediately after birth, the newborn has high levels of the mother's antibodies. Babies who are breastfed continue to receive antibodies via breast milk. Breast milk contains all five types of antibodies, including immunoglobulin A (IgA), immunoglobulin D (IgD), immunoglobulin E (IgE), IgG, and immunoglobulin M (IgM). This is called passive immunity because the mother is "passing" her antibodies to her child.