Food allergies, such as allergies to nuts or shellfish, are caused by a problem with your immune system. Food sensitivities or intolerances, such as reactions to wheat or milk, are caused by a problem with your digestive system. Food allergies can be life-threatening. A sensitivity or intolerance to a food can be uncomfortable, but it is rarely life-threatening. Also, even if you are sensitive to a food, you may still be able to enjoy it in small amounts.

The foods that most often cause allergic reactions in adults are the same for women and men. They include:

• Shellfish, such as shrimp, crayfish, lobster, and crab
• Peanuts
• Tree nuts, such as walnuts, cashews, and pecans
• Fish, such as salmon
• Milk
• Eggs
• Wheat
• Soybeans

The symptoms of an allergic reaction to a food usually develop within a few minutes to an hour after you eat the food. You may first feel itching in your mouth as you start to eat the food.

Other symptoms include:

• Stuffy, itchy nose
• Sneezing
• Itchy, watery eyes
• Swelling of the lips, face, tongue, throat, or other parts of your body
• Vomiting
• Diarrhea
• Stomach cramps
• Red, itchy skin or a rash

If you have food allergy symptoms shortly after eating, see your doctor or nurse. If possible, see your doctor while the allergic reaction is happening.

They can be. For some people, an allergic reaction to a food is uncomfortable but not serious. But for others, an allergic food reaction can lead to death. A life-threatening reaction caused by an allergy is called anaphylaxis.

For these people, even the smallest amount of exposure — eating a food or even touching someone who is eating the food — can be dangerous. If you have anaphylactic reactions to certain foods, your doctor may give you a prescription for injectable epinephrine. You need to carry this medicine with you at all times so that you or someone you are with can give you an emergency injection if needed.

Symptoms of anaphylaxis include:

• Hoarseness, throat tightness, or a lump in your throat
• Wheezing, chest tightness, or trouble breathing
• Rapid heart rate
• Dizziness, lightheadedness, or fainting
• Tingling in the hands, feet, lips, or scalp
• Cold, clammy, grayish, or bluish skin

Anaphylaxis is a medical emergency. If you or someone you know has any of these symptoms after eating something, call 911 right away.

Yes, other health problems can have some of the same symptoms as a food allergy. This can make it hard to know for sure whether you have a food allergy.

These health problems include:

• Food poisoning from contaminated food or foods with poisons, such as certain mushrooms
• Lactose intolerance
• Irritable bowel syndrome
• Reactions to large amounts of some food additives, such as monosodium glutamate (MSG, a flavor enhancer)

You do not need to avoid foods such as peanuts, milk, or eggs during pregnancy — unless you are allergic to any of these foods.

According to the American Academy of Pediatrics:

• Avoiding certain foods in pregnancy does not prevent food allergies in children
• Breastfeeding may prevent or delay food allergies
• Soy-based infant formula does not appear to prevent food allergy
• Delaying the introduction of solid foods beyond 4 to 6 months of age does not prevent food allergies. Some people have also thought that food allergies might be prevented if parents delayed giving their babies certain solid foods (such as fish, eggs, and milk). But current medical research does not support this idea.
• Recent research has shown that eating foods containing peanuts early in life may prevent a peanut allergy. If your infant has severe eczema, an egg allergy, or both, you may be able to give peanut-containing foods as early as 4 to 6 months of age to help prevent a peanut allergy. Check with your doctor or nurse before feeding your infant foods containing peanuts.

No. But sometimes babies may be allergic to something their mother eats, such as eggs, milk, or cheese. Babies who are highly sensitive usually react to the food within minutes. Babies who are less sensitive may still react to the food within four to 24 hours.

Symptoms may include:

• Diarrhea, vomiting, and/or green stools with mucus and/or blood
• Rash, eczema, dermatitis, hives, or dry skin
• Fussiness during and/or after feedings
• Inconsolable crying for long periods
• Sudden waking with discomfort
• Wheezing or coughing

These symptoms do not mean your baby is allergic to your milk, but rather to something you are eating. Talk with your baby’s doctor about any symptoms. If your baby ever has problems breathing, call 911 or go to your nearest emergency room.

Food allergies are often misunderstood.

"This disease is common and it has a dramatic impact not only on the lives of people who have the allergy but on the lives of anyone who cares about them," says Pamela Guerrerio, M.D., Ph.D. She is chief of the Food Allergy Research Unit of the Laboratory of Allergic Diseases at NIH's National Institute of Allergy and Infectious Diseases (NIAID). Dr. Guerrerio spoke to NIH MedlinePlus magazine about food allergies and research to help understand and potentially prevent them.

Allergic reactions

Allergic reactions can be scary if you've never had one before. If you have a mild allergic reaction, which could include hives, itching, sneezing, or a runny nose, you should take an antihistamine if it's available and monitor for more severe symptoms, Dr. Guerrerio said.

If you have a more severe reaction, you should seek immediate medical attention at an emergency room or call 911.

Severe reactions include difficulty breathing or low blood pressure, which can cause confusion, paleness, a weak pulse, or a lack of consciousness. Dr. Guerrerio said if the reaction is severe or if it involves multiple organ systems—for example the person has hives and is also vomiting—you should administer an EpiPen if you have one and call 911.

Once the initial reaction has been treated, you should see an allergist to get tested for a food allergy and discuss a course of action.

Development and diagnosis

Blood and skin tests can be helpful but are not enough to determine a food allergy. You must also have clinical symptoms when you eat the food.

Since these tests can result in false positives, "It's important that they are interpreted within the context of the patient's entire health picture," said Dr. Guerrerio.

Most allergies develop early in life, Dr. Guerrerio said, and food allergy development peaks around age 1.

It is uncommon for an allergy to develop later in life, though there are exceptions. The most common food allergies in adults include shellfish and nuts.

As people with a food allergy get older, they have increased chances of developing other allergic diseases or allergies. Sometimes people grow out of an allergy.

"It's really only with more research into food allergy that we're going to find better ways to diagnose, prevent, and treat the disease."

- Dr. Pamela Guerrerio

Food allergy studies

With so many unanswered questions surrounding food allergies, it's important that researchers continue to study them. "It's really only with more research into food allergy that we're going to find better ways to diagnose, prevent, and treat the disease," Dr. Guerrerio said.

A major, NIAID-funded study on peanut allergy prevention recently took place in the United Kingdom. This led to changes in food allergy guidelines from NIH. The study, Learning Early About Peanut Allergy (LEAP), looked at early exposure to peanuts in infants at high risk of developing a peanut allergy.

The study looked at 640 infants under the age of 1 who had either eczema, an egg allergy, or both. These conditions indicate a child is at high risk of developing a peanut allergy.

The children were split into two groups. The parents of the first group of children were told to have their child avoid peanut-containing foods completely until age 5. Parents of the second group were told to introduce peanuts into their child's diet immediately and children were to eat peanut-containing foods at least three times a week.

At age 5, the children had a peanut oral food challenge to see if they had developed a peanut allergy.

"The remarkable result was that the rate of peanut allergy was almost 80 percent lower in the group that had eaten peanut-containing products compared to the group that had avoided them," Dr. Guerrerio said.

Following the LEAP study and a followup study, LEAP-ON, an expert panel led by NIAID reviewed the available evidence and recommended that infants with a high risk of developing a peanut allergy be exposed to peanut-containing foods as early as 4 to 6 months old. The goal is to reduce the development of a peanut allergy.

Dr. Guerrerio said it's important that parents of children with a high risk for developing a peanut allergy speak to a provider before introducing peanuts into a child's diet. The provider can help determine if the child should first see an allergist for a food allergy test.

NIAID is also conducting research at the NIH Clinical Center in Bethesda, Maryland. The Institute is looking for participants for a study called "Natural History and Genetics of Food Allergy and Related Conditions."

The 10-year study aims to better understand the development of a food allergy and look at nutrition and growth in children with food allergies.

"Most of the major food allergens—such as milk, eggs, peanuts, soy, and wheat—are nutritionally dense. The concern is that children who have to avoid those foods because of their allergies will be at high risk for not getting enough nutrition. So we wanted to ask whether children with food allergy are indeed susceptible to nutritional deficiencies," Dr. Guerrerio said.

Eligible participants include anyone between the ages of 2 and 99 who has a food allergy confirmed by a doctor's testing. NIAID is also looking for healthy individuals (who do not have a food allergy) for the control group.