A delayed allergic reaction typically begins between two and three days after exposure to an antigen, rather than almost immediately. Such reactions can be observed with a number of allergens and can be tricky to diagnose and treat because doctors usually assume allergies are related to recent exposures. Some delays take even longer; in a condition called serum sickness, it may take a week or more for the patient to react to foreign proteins falsely tagged as dangerous by the body.
Patients experience a delayed allergic reaction when the response to an allergen is mediated by the leukocytes, the white blood cells. This secondary cellular response takes several days to manifest itself because the T cells and macrophages need time to start working. As a result, the patient might be exposed to an allergen and feel fine until several days later, when symptoms like rash and difficulty breathing develop. The intensity of the reaction can depend on immune health and the allergen.
Some drugs are associated with delayed allergic reaction, like certain antibiotics and antiserums used in treatment of disease. Patients who experience severe allergy symptoms and go to the doctor should make sure to cover their recent medical history. This includes not just what they ate, drank, or were exposed to in the last hours, but over the last week. The doctor might spot something that could have caused a delayed reaction and might be the cause of the symptoms.
It is possible to use allergy testing to check for a delayed allergic reaction. In this testing, the patient receives a series of challenges, exposures to an antigen through pricks or scratches in the skin. The doctor notes which challenges result in a response, and how long it takes for signs of a response to develop. Such testing can carry some risks for patients with extreme allergies, but the exposure is so minimal that the benefits of the testing in a controlled environment may outweigh the risks.
Recurrent allergies that do not seem to respond to testing or treatment could be an example of a delayed allergic reaction. The patient might be taking reasonable steps to identify allergens under the assumption that any reaction would occur immediately after exposure. Meanwhile, the culprit behind the delayed reaction might not be fingered because it doesn't cause the instantaneous response. These patients may need to meet with an allergy specialist to go over their history and explore testing options to find out what is making them sick.