One of the main characteristics that distinguishes food allergens from other allergens, such as pollen, is that you generally eat food allergens. Allergens can also enter your system through physical contact and via the air you breath, in the case of airborne allergens, but most food reactions occur as the result of eating an allergen.

The fact that eating an allergen triggers most reactions inspires two of the most common questions I field on a daily basis:

• How much of an allergen does it take to cause a reaction?
• How severe will my next reaction be?

Unfortunately, the answers to those two questions vary from one individual to another and can even vary for one individual over time. When someone asks me the amount of food required to trigger an allergy, I ask them to rephrase that question. A better question is “How much of a particular food is required to trigger my allergy?”

The point here is that everyone’s different. Each person who has a food allergy has a specific threshold — a specific dose of the food that sets the allergic reaction in motion. For some people that dose is incredibly small, for example 1/1000 of a peanut, but below that amount no reaction may occur. For others the threshold dose is quite large, for example five or six whole peanuts.

Several factors further complicate the process of coming up with a concrete answer that covers all cases:

• The dose that causes a reaction may change over time. In other words, you may be more or less sensitive to a food a couple months or years down the road. If you’re lucky, your sensitivity tapers off completely and you outgrow your allergy.

If you’re not so lucky, your sensitivity can increase over time (unfortunately a common occurrence with some allergies), and then you may react to lower and lower doses and your reactions may increase in severity.

• You may be more sensitive when your immune system is revved up. A recent food reaction or a bad pollen season, for example, can lower your threshold.

• Your tolerance level can fluctuate. You may be able to tolerate a given amount of food very intermittently, but as you expose yourself to it more regularly, you begin to react to it.

• Some evidence suggests that in some people, exposure to even a small amount of food, even though the exposure doesn’t cause an obvious reaction, makes it harder to outgrow the allergy. The idea here is that these small exposures rev up the immune system and prolong the allergy, sometimes even making the allergy stronger and more dangerous.

Patients who can tolerate small amounts of a known allergen on an occasional basis may benefit from a rotation diet. For example, if you have milk allergy and the bread you enjoy contains tiny amounts of milk, you may be okay eating that bread once a week.

Don’t try this on your own; consult your doctor first. Exposing yourself to even a small amount of the offending food could result in a severe reaction, and even if it doesn’t, the exposure could compromise your chances of outgrowing your allergy.

Once you’ve experienced an allergic reaction, the first question is often “How bad will the next one be?” The rule of thumb for predicting the severity of your next reaction is that the higher your sensitivity and the more allergen you ingest above and beyond your threshold the worse the reaction.

In other words, if you eat an oatmeal cookie that shared a spatula with a peanut butter cookie and you have a mild reaction, you can very likely expect a much more severe reaction if you gobble up a cookie with five peanuts in it. Like most rules of thumb, this one begins to break down upon closer examination.

Allergies tend to defy rules and common logic and toss in variables that call predictions into question:

• Your threshold can change. Say you eat 25/1000 of a peanut on a day when your threshold is 1/1000 of a peanut. On another day, you eat 40/1000 of a peanut when your threshold is 35/1000 of a peanut. You can expect a more severe reaction in the first case, even though you consume much more peanut in the second case.

• Your reactivity can change. If you’ve had a recent reaction or you’re in the midst of a bad pollen season, you may react more strongly than normal.

• Having asthma can increase the severity of reactions, especially reactions that cause breathing problems. In general, the worse the asthma, the more severe the reaction. In other words, you can expect less severe reactions on days when your asthma is under control. (Asthma and food allergies commonly occur in the same people.)

People (including many doctors) often say that severity increases with each subsequent reaction. However, while this may occur, allergies follow no predictable pattern. You really can’t predict the severity of your next reaction.

The best approach is to play it safe and team up with your doctor to track your allergy’s ups and downs. Most importantly, always assume that a worse reaction may occur and be prepared to treat it.