hypersensitivity reaction
DEFINITION:
Overreaction of the immune system (defense against infection) to an antigen (protein recognized as foreign). Hypersensitivity reactions occur only on second or subsequent exposures to particular antigens, after the first exposure has sensitized the immune system. Such reactions have the same mechanisms as those of protective immunity. However, while the latter protect against disease, hypersensitivity reactions lead to tissue damage and disease.
Hypersensitivity is closely related to allergy, except that only one of the four main types of hypersensitivity reaction (type I) is closely associated with allergic illnesses.
Types
The four main types of hypersensitivity reaction are as follows.
Type I - This type is also called immediate or anaphylactic hypersensitivity. After a first exposure to an antigen (which may be a harmless substance such as grass pollen), antibodies (substances that can recognize and bind to the antigen) are formed; these antibodies coat cells called mast cells in various tissues. On second exposure, the antigen and antibodies combine, causing the mast cells to disintegrate and release various chemicals that cause the symptoms of asthma, allergic rhinitis (hay fever), urticaria (hives), anaphylactic shock, or other illnesses of an allergic nature.
Type II - In this type, antibodies are formed that bind to antigens bound to cell surfaces, leading to possible destruction of the cells. Type II reactions may be responsible for certain autoimmune disorders (in which antibodies attack the body's own tissues) and for some cases of red blood cell destruction (hemolysis) triggered by certain drugs.
Type III - Antibodies combine with antigens to form particles called immunocomplexes, which can lodge in various tissues and activate further immune system responses, leading to tissue damage. This type of hypersensitivity reaction is responsible for serum sickness, for allergic alveolitis (a lung disease caused by exposure to the spores of certain fungi), and for the large swellings that sometimes form after booster vaccinations.
Type IV - This type is also called delayed hypersensitivity. In type IV, sensitized T lymphocytes (a class of white blood cell and an important component of the immune system) bind to antigens and subsequently release chemicals called lymphokines, which promote an inflammatory reaction. Type IV reactions are responsible for contact dermatitis and the rash of measles; they are important in the body's defense against tuberculosis and may also play a part in some "allergic" reactions to drugs.
Treatment
Effective treatment of a hypersensitivity reaction depends on its type, cause, and severity. When possible, exposure to the offending antigen should be avoided.
hypersensitivity reaction
DEFINITION:
Overreaction of the immune system (defense against infection) to an antigen (protein recognized as foreign). Hypersensitivity reactions occur only on second or subsequent exposures to particular antigens, after the first exposure has sensitized the immune system. Such reactions have the same mechanisms as those of protective immunity. However, while the latter protect against disease, hypersensitivity reactions lead to tissue damage and disease.
Hypersensitivity is closely related to allergy, except that only one of the four main types of hypersensitivity reaction (type I) is closely associated with allergic illnesses.
Types
The four main types of hypersensitivity reaction are as follows.
Type I - This type is also called immediate or anaphylactic hypersensitivity. After a first exposure to an antigen (which may be a harmless substance such as grass pollen), antibodies (substances that can recognize and bind to the antigen) are formed; these antibodies coat cells called mast cells in various tissues. On second exposure, the antigen and antibodies combine, causing the mast cells to disintegrate and release various chemicals that cause the symptoms of asthma, allergic rhinitis (hay fever), urticaria (hives), anaphylactic shock, or other illnesses of an allergic nature.
Type II - In this type, antibodies are formed that bind to antigens bound to cell surfaces, leading to possible destruction of the cells. Type II reactions may be responsible for certain autoimmune disorders (in which antibodies attack the body's own tissues) and for some cases of red blood cell destruction (hemolysis) triggered by certain drugs.
Type III - Antibodies combine with antigens to form particles called immunocomplexes, which can lodge in various tissues and activate further immune system responses, leading to tissue damage. This type of hypersensitivity reaction is responsible for serum sickness, for allergic alveolitis (a lung disease caused by exposure to the spores of certain fungi), and for the large swellings that sometimes form after booster vaccinations.
Type IV - This type is also called delayed hypersensitivity. In type IV, sensitized T lymphocytes (a class of white blood cell and an important component of the immune system) bind to antigens and subsequently release chemicals called lymphokines, which promote an inflammatory reaction. Type IV reactions are responsible for contact dermatitis and the rash of measles; they are important in the body's defense against tuberculosis and may also play a part in some "allergic" reactions to drugs.
Treatment
Effective treatment of a hypersensitivity reaction depends on its type, cause, and severity. When possible, exposure to the offending antigen should be avoided.
