What Is the Function of a Delayed Hypersensitivity T Cell?
Delayed hypersensitivity reactions (also known as Type IV hypersensitivity) are inflammatory reactions initiated by immune cells called mononuclear leukocytes. The reaction occurs once the mononuclear leukocytes are exposed to an antigen. Following this exposure, there is a delayed (48 to 72 hours) response whereby T cells that recognize the antigen can do one of two things: 1) help the mononuclear leukocytes destroy the antigen (helper T cells) or 2) the T cells directly destroy the antigen (killer T cells).
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What is Hypersensitivity?
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Hypersensitivity is a term used to describe an exaggerated immune reaction to an agent (pathogen) that is harmful to the host. It is often confused with the term "allergy," which should only be used to describe Type I hypersensitivity.
Pathogens that Elicit Delayed Hypersensitivity
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Delayed hypersensitivity reactions are protective against pathogens that invade cells. These pathogens include mycobacteria, fungi, and some parasites.
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Common Delayed Hypersensitivity Reactions
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Many skin rashes that occur after contact with an offending agent are the result of delayed hypersensitivity. Offending agents may include poison oak or ivy, heavy metals in jewelry, or cosmetic products.
In addition, a red induration after exposure to the purified protein derivative (PPD) in a tuberculosis skin test is a delayed hypersensitivity response that indicates the presence of the tuberculosis in the body.
Impairments in Delayed Hypersensitivity
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Individuals with low immune function may have deficiencies in delayed hypersensitivity reactions. This is especially true in individuals with AIDS because this disease severely impairs the response of helper T cells. Therefore, AIDS patients are at high risk of developing opportunistic infections with Mycobacteria tuberculosis and other pathogens. Additionally, individuals with impaired delayed hypersensitivity will not demonstrate common hypersensitivity reactions such as those mentioned above.
Other Types of Hypersensitivity Reactions
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Other types of hypersensitivity reactions are:
Immediate (Type I) hypersensitivity, which commonly manifests as hives, eczema, red and itching eyes and/or nose, and asthma.
Cytotoxic (Type II) hypersensitivity, which occurs with non-matched blood transfusions and some drug reactions.
Immmune-complex (Type III) hypersensitivity, which is the cause for some autoimmune disorders like rheumatoid arthritis and lupus.
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