Hypersensitivity reactions, or allergies, are damaging reactions of the body's immune system to a specific substance. The body normally secretes antibodies, cells produced by the bone marrow, and other cells to fight against foreign substances. Once a foreign substance enters the body, specific antibodies are produced to neutralize or kill them. These antibodies are capable of recognizing the foreign substance on subsequent exposures. Some individuals, however, become sensitive to a particular substance, leading to the occurrence of hypersensitivity reactions during the second contact and every time contact with the offending substance, or allergen, happens thereafter.
Individuals with a family history of allergies have higher risks of developing hypersensitivity reactions. Environmental factors can also bring about the condition, such as early exposure to the offending substance or substances. Season change also often trigger many hypersensitivity reactions in susceptible individuals, especially children with allergies to pollen and house dusts.
There are four known types of hypersensitivity reactions. The first type is immediate hypersensitivity reaction, which often presents allergy symptoms within minutes after exposure. It occurs often to allergens such as animal dander, house dusts and foods, such as peanuts and eggs. Conditions exhibiting type I reactions include runny nose or allergic rhinitis, and urticaria, characterized by redness, itchiness and swelling of the skin. Anaphylactic shock, also under type I, is a fatal reaction that causes swallowing and breathing difficulties, fainting, low blood pressure and can even lead to death if not managed early. Anaphylactic shock is frequently caused by bee stings and drug administration, such as antibiotics and hormones.
Type II reactions are antibody mediated reactions or cytotoxic hypersensitivity reactions. These generally occur when tissues or organs in the body are destroyed due to the action of antibodies fighting the offending substances. Conditions under type II include hyperthyroidism or excessive production of thyroid hormones; drug-induced hemolytic anemia, or destruction of red blood cells as a reaction to a drug; and Goodpasture syndrome, a rare autoimmune disease affecting the lungs and kidneys. Acute rheumatic fever is another type II reaction. It manifests with inflammation of the heart muscles after a streptococcus bacteria infection.
In immune complex mediated reactions, or type III, a foreign-substance-and-antibody complex usually deposits on an organ, causing damage to the organ in the process. Examples of conditions under type III include rheumatoid arthritis, characterized by stiffness and joint pains; and systemic lupus erythematosus, an autoimmune disease manifested with a butterfly rash on the face, fatigue and inflammation of the kidneys. Poststreptococcal glomerulonephritis, also under type III, is a deposit of antibody and streptococcus bacteria complex in the kidney, usually after a sore throat. It often results in kidney inflammation manifested by presence of blood in the urine, less urine output, fever and edema.
Delayed hypersensitivity, or the 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 test for tuberculosis. A small amount of antigen is injected in a small area of the skin, which typically becomes red and swollen within several hours or up to 72 hours later if the individual has been exposed to the organism causing tuberculosis.