All adverse reactions to drugs are classified into predictable and unpredictable. The first group is associated with pharmacological actions of the medication in otherwise normal individuals. The second group of adverse reactions are connected to an individual's immunological response. Drug allergy is a member of the second group of adverse reactions to drugs - unpredictable ones.
We need to differentiate two terms, drug hypersensitivity and drug allergy. The first term refers to reproducible symptoms and signs which are associated with exposure to the specific drug when the dose of the drug is normally tolerated by individuals who are not hypersensitive. The other term, drug allergy, is immunologically mediated drug hypersensitivity reaction with severe symptoms and signs. It develops even when there is an intake of very small doses of the drug.
Drug Allergy - Development
Allergy to a drug develops once the body gets exposed to a medication and then produces specific antibodies. After repeated exposure antibodies and disease-fighting cells initiated by a specific body reaction induce a sequence of immunological reactions which eventually leads to a typical allergic reaction.
Antibodies are blamed for the release of chemicals called histamines, the ones that initiate symptoms and signs of an allergic reaction.
It is essential to memorize that during the initial exposure to the drug the body gets sensitized and each subsequent exposure is accompanied by immune response.
Direct drug allergies develop after one has been exposed to different drugs. The examples are allergy to penicillins, some vaccines, blood transfusions, insulin etc. Indirect drug alllergies, on the other hand, do not engage the immune system in release of histamine, they do it by themselves. This is the case with Aspirin and anti-inflammatory drugs, morphine and some other opiates as well as intravenous contract dyes.
In individuals suffering from drug allergy, exposure to even small amounts of the medication capable of triggering allergy reaction is responsible for sometimes even life-threatening condition.
Even though one may develop allergy to different and almost any drug, the most common drugs that generally cause allergic reaction among people include antibiotics (penicillin, cephalosporins and sulfonamides), ACE inhibitors, opiates (codeine, morphine etc.) and non-steroidal anti-inflammatory drugs.
Drug Allergy - Host Factors
Some drug allergies are less frequent among children and elderly people. This is explained by immaturity/involution of their immune system. Scientists believe that both genetics and environmental factors contribute to drug allergy.
Furthermore, in some individuals the disease state may affect the development of drug allergy. And finally, people who have already developed some drug allergy are highly likely to develop an allergic reaction to another drug. This is why doctors pay close attention when introducing the new drug in such people and pay special attention to cross-sensitization.