Allergies are harmful reactions of the immune system to a specific substance. There are four types of hypersensitivity reactions, including immediate, antibody-mediated, immune complex-mediated, and delayed hypersensitivity. Individuals with a family history of allergies are at an increased risk of developing hypersensitivity reactions.
Hypersensitivity reactions, or allergies, are harmful reactions of the body’s immune system to a specific substance. The body normally secretes antibodies, cells made by the bone marrow, and other cells to fight foreign substances. Once a foreign substance enters the body, specific antibodies are produced to neutralize or kill them. These antibodies are able to recognize the foreign substance on subsequent exposures. Some individuals, however, become sensitive to a particular substance, leading to hypersensitivity reactions occurring during the second contact and whenever contact with the offending substance, or allergen, occurs thereafter.
Individuals with a family history of allergies are at increased risk of developing hypersensitivity reactions. Environmental factors may also determine the condition, such as early exposure to the offending substance or substances. Even the change of season often triggers many hypersensitivity reactions in predisposed individuals, especially children with allergies to pollen and house dust.
There are four known types of hypersensitivity reactions. The first type is the immediate hypersensitivity reaction, which often presents allergy symptoms within minutes of exposure. It often presents itself to allergens such as animal dander, house dusts, and foods, such as peanuts and eggs. Conditions showing type I reactions include runny nose or allergic rhinitis and hives, characterized by redness, itching, and swelling of the skin. Anaphylactic shock, even type I, is a fatal reaction that causes swallowing and breathing difficulties, fainting, hypotension, and can even lead to death if not managed early. Anaphylactic shock is often caused by bee stings and the administration of drugs, such as antibiotics and hormones.
Type II reactions are antibody-mediated reactions or cytotoxic hypersensitivity reactions. These generally occur when body tissues or organs are destroyed due to the action of antibodies that fight the offending substances. Type II conditions include hyperthyroidism or excessive production of thyroid hormones; drug-induced hemolytic anemia or red blood cell destruction as a reaction to a drug; and Goodpasture’s syndrome, a rare autoimmune disease that affects the lungs and kidneys. Acute rheumatic fever is another type II reaction. It occurs with inflammation of the heart muscles after an infection with streptococcal bacteria.
In immune complex-mediated, or type III, reactions, a complex of foreign substances and antibodies usually deposits on an organ, causing damage to the organ in the process. Examples of type III conditions include rheumatoid arthritis, characterized by joint stiffness and pain; and systemic lupus erythematosus, an autoimmune disease that manifests as a butterfly rash on the face, fatigue, and inflammation of the kidneys. Poststreptococcal glomerulonephritis, also type III, is a deposit of antibodies and streptococcus bacteria complex in the kidney, usually after a sore throat. It often causes kidney inflammation manifested by blood in the urine, decreased urine output, fever, and edema.
Delayed hypersensitivity, or a type IV reaction, usually occurs several hours, or even more than a day, after exposure to the foreign substance or organism. The best example is the tuberculosis test. A small amount of antigen is injected into a small area of skin, which typically becomes red and swollen within hours, or up to 72 hours afterward if the individual has been exposed to the organism that causes tuberculosis.
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