What Is an Allergic Reaction?

Content provided by the Faculty of the Harvard Medical School
Excerpted from a Harvard Special Health Report

What Is an Allergic Reaction?

Allergies belong to a category of immune system responses called hypersensitivity responses. In fact, if the immune system is not involved, it is not, medically speaking, an allergy.

People often confuse reactions to irritants in the environment — like a runny nose irritated by cold air, smoke, or perfumes — with a true allergic reaction such as hay fever, in which cells of the immune system respond to an allergen such as ragweed. Some people develop an itchy, red rash after contact with certain chemicals such as a harsh laundry detergent, but if the immune system isn't involved, it is an irritation, not an allergy. Likewise, someone who suffers from bloating and diarrhea after drinking milk or eating dairy products may have an intolerance, not an allergy, to lactose, a natural sugar found in cow's milk, because they lack the enzymes needed to digest lactose (see "Tolerating intolerances," below).

An allergic reaction can happen almost instantaneously, or it can happen after hours or days. This defines the type of response as either an immediate or delayed hypersensitivity reaction.

Innate immunity, the type of immunity everyone is born with, is the body's first activated response to germs (pathogens). Innate immunity triggers a swift inflammatory response to a "non-self" invader. Response time is typically within 12 minutes of exposure to the allergen. The innate response then fires up the next line of defense, the adaptive immune response. The adaptive immune system, which requires "schooling" in our early years so it can recognize pathogens when it meets them, takes longer to become activated — but once it is, it is immensely powerful.

Tolerating intolerances

People often confuse intolerance with allergy. One way to define intolerance is by saying what it isn't: it isn't an overzealous immune system response. Typically it is a response to a chemical or the consequence of an insufficiency of a natural compound. For some people, the caffeine, theobromine, and methylxanthine in tea, coffee, chocolate, and cocoa cause symptoms of acid reflux, jitteriness, or insomnia. The chemicals in red wine can cause migraines in some people; monosodium glutamate (MSG), sometimes present in Chinese dishes or processed foods, bothers others — although the latter is not as common as people tend to think it is. Other people suffer discomfort from milk products because their bodies produce little or no lactase (the enzyme that breaks down lactose in milk), a condition that can worsen with age.

As with allergies, avoidance is the first line of treatment for intolerances. Specific strategies may help alleviate the intolerance. For example, for lactose intolerance, you may be able to boost your lactase levels by taking nonprescription supplements available in retail stores, although the benefits are variable. If you suffer from digestive disturbances, check with your doctor rather than attempting self-diagnosis.

Immediate hypersensitivity reactions

In allergic individuals, cells in the immune system that normally help fend off germs can become overactive and respond inappropriately and very swiftly to otherwise harmless substances known as allergens.

Keys to this process are the helper T cells 1 (Th1) and 2 (Th2). These white blood cells circulate in the bloodstream and alert other immune system players that the body may be under attack from invading germs. Th1 cells handle certain types of viral and bacterial infection, while Th2 cells help in eliminating certain parasites. Research has identified a third type of helper T cell called Th17, which appears to have a role in the allergic response.

In allergic diseases, this process goes awry in at least two ways. First, Th2 cells dominate, meaning they are more likely to respond than Th1 cells. Second, the body mounts these Th2 responses to substances that are not actually harmful, such as pollen and animal dander. In response, Th2 cells produce substances and recruit other cells — mast cells and eosinophils — that become involved in an allergic reaction. The proteins produced by the Th2 cell, called cytokines, orchestrate the allergic response.

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Pollen counts are highest on dry, hot, and windy mornings, between 5 and 10 o'clock.

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