A drug allergy is caused when the immune system mistakenly identifies a drug as a harmful substance, as if it were a viral or bacterial infection.
The drug allefrgy usually occurs after the immune system has been sensitized to a drug. This means that the first time the drug is taken the immune system detects it as a harmful substance and develops an antibody specific to the drug. The next time this drug is taken, these specific antibodies flag the drug and direct the immune system attacks on the substance. Chemicals released by this activity cause the signs and symptoms associated with an allergic reaction.
Some allergic reactions may result from drugs binding directly to a certain type of immune system white blood cell called a T cell. This sets in motion the release of chemicals that cause an allergic reaction.
In such cases an allergic reaction can occur the first time the drug is taken.
Opiates for treating pain, local anaesthetics such as procaine and dyes used during imaging tests can also cause allergy reactions.
4 Making a Diagnosis
The following may be used for the diagnosis of drug allergy.
A physical examination and a medical history, showing details about the onset of symptom and time when the drug was taken.
Skin tests in which an allergist or nurse administers a small dose of the suspected drug to the skin either with a tiny needle that scratches the skin, an injection or a patch. A positive reaction to a test will cause a red, itchy, raised bump. A positive result almost always indicates an allergy. A negative result is more difficult to interpret because of differences in reliability of tests.
Blood tests to rule out other possible causes of signs and symptoms.
Treatments of drug allergy are divided into:
treatment for present allergy symptoms and
treatment that may enable a person to take an allergy-causing drug if it is medically necessary.
Treatments for prevent drug allergy symptoms include:
Withdrawal of the drug.
Using antihistamines or over-the-counter antihistamines such as diphenhydramine (Benedryl) that can block immune system chemicals activated during an allergic reaction.
Using oral or injected corticosteroids to treat inflammation.
Treating anaphylactic shock by injecting epeinephrine.
Two strategies can be used when taking allergy-causing drugs. They include:
Graded challenge - a procedure in which a person receives four to five doses of a suspected drug, starting with a small dose and increasing to the desitred dose. If the person reaches the therapeutic dose without a reaction, the doctor will conclude that the person is not allergic to the drug. It is performed when a drug allergy is uncertain and a doctor judges that an allergy is unlikely.
Drug desensitization - this is required before giving a patient a drug that causes an allergy. A patient receives a very small doser every 15 to 30 minutes over several hours or days. If a person reaches the desired dosage with no reaction, then treatment can be continued.
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