Why do some allegens produce runny nose while others diarrhea?

Different allergens produce different symptoms because they target different parts of the immune system. Some allergens, such as pollen and dust mites, are inhaled and cause respiratory symptoms such as a runny nose, sneezing, and itchy eyes. These allergens are typically recognized by antibodies called immunoglobulin E (IgE), which bind to the allergens and trigger the release of histamine and other inflammatory mediators from mast cells and basophils. These mediators cause the symptoms of an allergic reaction.

Other allergens, such as certain foods and medications, are ingested and cause gastrointestinal symptoms such as diarrhea, nausea, and vomiting. These allergens are typically recognized by antibodies called immunoglobulin G (IgG) or immunoglobulin A (IgA), which bind to the allergens and trigger the release of inflammatory mediators from immune cells in the gut. These mediators cause the symptoms of an allergic reaction.

In some cases, an allergen can cause both respiratory and gastrointestinal symptoms. This is because some allergens can be both inhaled and ingested, or they can cross-react with allergens from different sources. For example, someone who is allergic to pollen may also be allergic to certain fruits and vegetables, such as apples and carrots, because these foods contain proteins that are similar to the proteins found in pollen.