An overactive immune system. If you are born with certain genes, your immune system may react to substances in the environment that are normally harmless. These substances are called allergens.
The immune system is a system of many biological structures and processes within an organism that protects against disease. To function properly, an immune system must detect a wide variety of agents, known as pathogens, from viruses to parasitic worms, and distinguish them from the organism's own healthy tissue. The principle behind vaccination (also called immunization) is to introduce an antigen from a pathogen in order to stimulate the immune system and develop specific immunity against that particular pathogen without causing disease associated with that organism. Introduction T he immune system is a network ofcells, tissues *, and organs that worktogether to defend the body against attacksby “foreign” invaders. These are primarily microbes (germs)—tiny, infection-causing organisms such as bacteria, viruses, parasites, and fungi. In other cases, the immune system responds to a seemingly harmless foreign substance such as ragweed pollen. The result is allergy, and this kind of antigen is called an allergen. The Structure of the Immune System T he organs of the immune system are positioned throughout the body. The soldiers of the immune system are white blood cells, including T and B lymphocytes, which originate in the bone marrow from hematopoietic stem cells. Every day the body comes into contact with many organisms such as bacteria, viruses, and parasites. When a healthy individual is infected, the body responds by activating a variety of immune cells. Initially, invading bacteria or viruses are engulfed by an antigen presenting cell (APC), and their component proteins (antigens) are cut into pieces and displayed on the cell's surface. The immune system recognizes and destroys substances that contain antigens. Your body's cells have. that are antigens. These include a group of antigens called. Your immune system learns to see these antigens as normal and usually does not react against them. 1  proteins. Innate immunity also comes in a protein chemical form, called innate humoral immunity. Examples include the body's complement system and substances called interferon and interleukin-1 (which causes fever). If an antigen gets past these barriers, it is attacked and destroyed by other parts of the immune system. Transplant rejection occurs when transplanted tissue is rejected by the recipient's immune system, which destroys the transplanted tissue. Transplant rejection can be lessened by determining the molecular similitude between donor and recipient and by use of immunosuppressant drugs after transplant.  Disorders of the Immune System. Your immune system is your body's defense against infections and other harmful invaders. Without it, illnesses from bacteria or viruses, for example, would be constant. Your immune system is made up of special cells, tissues, and organs that work together to protect you. When your immune system doesn't work the way it should, it is called an immune system or immunodeficiency disorder. You may: Be born with a weak immune system. This is called primary immune deficiency. The lymph, or lymphatic, system is a major part of the immune system. It's a network of lymph nodes and vessels. Lymphatic vessels are thin tubes that branch, like blood vessels, throughout the body. They carry a clear fluid called lymph. Lymph contains tissue fluid, waste products, and immune system cells. When your immune system doesn't work the way it should, it is called an immune system or immunodeficiency disorder. You may: Be born with a weak immune system. This is called primary immune deficiency. An overactive immune system. If you are born with certain genes, your immune system may react to substances in the environment that are normally harmless. These substances are called allergens. Having an allergic reaction is the most common example of an overactive immune system. When your immune system doesn't work the way it should, it is called an immune system or immunodeficiency disorder. You may: Be born with a weak immune system. This is called primary immune deficiency. How the Immune System Works TOP. The immune system is a complex biological system in the human body. It is made up of cells, tissues, and organs that work constantly to deal with infectious and other foreign materials, such as allergens or tumors, that may be harmful to the body. These materials carry antigens. 
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why is immune rejection an example of a healthy immune system