Learn how babies develop food allergies

  • Learn how a baby becomes allergic to a food.
  • Cow’s milk protein allergy—the most common allergy in babies.

Your baby’s digestive system is still immature and developing every day. That’s why babies, although uncommon, have a higher chance of having an allergy to their infant formula, and, in rare cases, the cow milk protein Mom consumes passing through to her breastmilk. In addition, most of the immune cells within our bodies are found inside the digestive tract. Because of these two facts, proteins, like the ones found in cow and soy milk formulas, may be seen by a baby’s immune system as a protein that doesn’t belong. This is called “sensitization.” As a result, the next time the baby eats these proteins, her immune system may react by trying to fight the proteins, which leads to signs or symptoms of an allergic reaction.

Here’s how it works:

  1. A food allergy develops when a baby’s immune system becomes “sensitized” to an allergen or protein in a certain food. This may happen the first time the baby has a bite of the food, but can also come later.
  2. Once sensitized, the baby‘s body makes antibodies—little proteins that “lock” onto foreign invaders—that are ready to react to that food the next time it’s eaten. 
  3. These antibodies then trigger immune cells to make histamine, a chemical that causes allergy symptoms—rash, wheezing, diarrhea, etc.
Cow’s milk protein allergy
Cow’s milk protein allergy is the most common allergy in infancy affecting 2–7% of babies. Symptoms can appear from minutes to hours after the baby drinks or eats the protein. Infant formula is the most common culprit in early infancy. Though uncommon, babies can react to the cow’s milk protein that Mom has in her diet, which can be passed through to her breastmilk.

Cow’s milk protein allergy reactions or symptoms can be confused with other conditions that are commonly seen in babieslike colic. This can make it hard to spot the cause of baby’s discomfort. If you think your baby may have a food allergy, talk to your pediatrician as soon as possible to see if allergy testing might be a good idea.

Your baby’s pediatrician may recommend allergy testing, but usually will initiate a change in infant formulas to a hypoallergenic formula prior to any additional testing. 

What to expect at your visit:

  • The doctor will ask about your baby’s medical history.
  • Be ready to talk about what she’s eaten in the past few days and weeks, including how often she has had symptoms, how severe they have been, and the time between eating the food and the reaction.
  • A physical examination.
  • Possible allergy testing. Based on your baby’s current condition and history, the doctor will decide if any testing is necessary.

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