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Delayed hypersensitivity is difficult to understand without a working definition of hypersensitive. In the medical sense, as opposed to an emotional meaning, hypersensitivity is strong bodily reaction to some form of antigen. An antigen is a substance introduced to the body or one perceived as foreign by the body. The body reacts to the antigen in a perceivable way, such as by development of a rash, getting asthma, developing allergy symptoms or showing signs of contagion from illness, but sometimes this reaction doesn’t occur immediately and is delayed hypersensitivity.
Hypersensitivity is often described in four classes/types. The forth class is delayed hypersensitivity. Usually this delay is due to the way the body reacts. The extra time taken for reaction tends to mean T cells are involved in response.
It’s easy to think of examples of delayed hypersensitivity. When people touch poison oak or ivy, it may take at least a day before the skin reacts. In this time, T cells have gone to work to create the rash that is the hallmark of exposure to poison oak or ivy antigens. Most people exposed to illnesses also don’t get them right away, but may develop them in a few days or weeks.
Immediate hypersensitivity reactions are often due to allergies. A person allergic to peanuts could develop severe allergies within seconds of consuming a single peanut. This quick histamine response can be very dangerous, and usually is not delayed in any way. Immediate treatment is needed to avoid anaphylactic shock.
One complicated aspect of delayed hypersensitivity may be a chronic condition or a constant reaction to antigens that takes a few days to fully express. Yet when expression begins to occur, it appears to be an immediate response. An example of this is rejection of organs.
When a person first receives a donor organ, it may not seem that rejection is occurring, and overt expression of rejection could take several days, while T cells marshal their forces. Once that rejection establishes itself, it may still be a delayed reaction, though clearly the organ begins to fail. While the reaction is delayed hypersensitivity, organ failure occurs concurrently, making it seem as though the reaction is to the present state instead of a process that began a few days ago. To avoid this, anti-rejection medications are often given before T cell response can occur, leading to fewer cases of organ failure.
Delayed hypersensitivity is not always a negative. When folks undergo tuberculin skin testing they have to get the results checked about a week later for evidence of tuberculosis exposure. The delayed reaction is simply accepted as the way the body responds to the test and the antigen; it’s neither good nor bad, provided people go back to the doctor to have results interpreted. Additionally, as with organ transplants, it may be possible to medically intervene between exposure and reaction to certain substances, reducing chances of reaction severity.