An allergy to peanuts is one of the most serious allergies, and is estimated to be involved in as many as 80% of fatal or near fatal allergic reactions every year. In the United States alone, 1.5 million people have a peanut allergy, and this rate is climbing, as it is in other First World countries. The exact cause of peanut allergy is unknown, although the biological process that creates an allergic reaction is well understood. No matter what causes it, a peanut allergy can be fatal to the sufferer, so be respectful of people who say they are allergic to peanuts and try to be conscientious around them; sometimes even being touched by someone who has handled peanuts will stimulate a reaction.

Like other allergies, a peanut allergy is an abnormal immune system response. The body decides that the proteins peanuts contain pose an immunological threat, and it generates Immunoglobulin E (IgE), an antibody, to fight peanuts the next time they appear in the body. Sometimes it takes multiple exposures to peanuts to develop IgE. When someone with a peanut allergy eats peanuts, it triggers the formation of histamines in the body to fight the peanuts, causing an allergic reaction.

In a mild peanut allergy, sometimes differentiated as intolerance, consumption of peanuts can be accompanied by intestinal distress and heart burn. In the case of a serious peanut allergy, the victim may develop hives, skin rashes, difficulty breathing, accelerated heart rate, and in an extreme case, anaphylaxis, which is a state of unconsciousness that can rapidly lead to coma. The patient must be immediately treated with epinephrine to suppress the immune system response, which is why many people with severe allergies carry allergy kits.

Studies have shown that young children are more likely to develop peanut allergies than adults. If a child is not exposed to peanuts before the age of four, it is highly probable that he or she will not develop an allergy. In some instances, children have also grown out of peanut allergies, although this should be determined by a medical professional. Family history is a major risk factor; children of parents who are allergic to peanuts are often allergic as well. If someone suspects a peanut allergy, allergy testing can be conducted to confirm it. Patients can either undergo skin testing, which directly embeds proteins under the skin, causing a rash if the person is allergic, or a blood test which looks for IgE in the blood.

The growing incidence of peanut allergy in first world countries has led doctors to suspect that humans may be altering their immune systems by eradicating diseases and keeping their environments too clean. The absence of threats in the immune system's natural environment may lead it to do strange things, like develop adverse reactions to ordinary foods. In addition, the heavy use of peanut products in foods may be contributing, as it is hard to avoid exposing young children to peanut products, which are found literally everywhere; oils, most processed foods, skin care products, and in some plastics, to start with.