Formula allergies are an immune response that occurs when an infant's immune system mistakenly views milk (or soy) protein as a danger and makes antibodies to fight it off, similar to the way the immune system fights off a virus. Repeated exposure results in the antibodies triggering a release of histamine, which is what causes the signs of an allergic reaction. According to Children's Hospital Boston, allergic symptoms may appear within minutes of ingesting the formula or not until days later. The Nemours Foundation states that most children outgrow milk allergies by the age of 5 years.
Gastrointestinal Signs
Gastrointestinal signs, such as severe spitting up, projectile vomiting, abdominal pain, diarrhea and bloody stool may develop soon after feeding or may take up to seven days to appear. Blood and mucus in the stool may be obvious or may go unnoticed by parents. Fecal testing may show trace amounts of blood, and chronic bleeding may lead to anemia in severe cases. While diarrhea is more common in cases of allergies to infant formula, constipation may also develop. There may be a rash around the anus and abdominal bloating. Diarrhea, vomiting and intestinal inflammation may lead to poor growth.
Respiratory Signs
Respiratory signs usually appear more quickly than gastrointestinal symptoms. These may include wheezing, persistent coughing, asthma, sinus congestion, snoring and frequent ear infections caused by fluid buildup in the sinuses. Some infants with an allergy to formula experience frequent or chronic cold-like symptoms, such as watery or red eyes, sneezing, a runny or stuffy nose and chest congestion.
Eczema and Skin Rashes
Eczema and other skin rashes are a common sign of an allergy to infant formula, especially to the protein in cow's milk. The dry, scaly patches of eczema may occur on the face, arms or legs. Hives or a sandpaper-like rash may develop soon after ingesting the formula. Puffy eyes and dark circles under the eyes may also occur.
Colic and Irritability
Allergies to infant formula may present as general fussiness after feeding, excessive gas and abdominal cramping. Pediatrician Alan Greene states that infants with formula allergies may be fussier than other babies, with many developing colic after consuming the offending food. Colic is also common in babies with lactose intolerance, which is not the same as a true allergy to infant formula. Lactose intolerance is an inability to digest lactose, a type of sugar found in dairy products. A formula intolerance does not affect the immune system.
Anaphylactic Signs
Severe allergic reactions can occur within minutes of ingestion and may be life-threatening. Symptoms of anaphylaxis may include swelling of the mouth, lips, or tongue, difficulty breathing, wheezing and lowered blood pressure. These symptoms require emergency medical attention. The Nemours Foundation states that anaphylaxis is more likely to occur with other food allergies than with an allergy to milk protein.
