Why do some people have allergic reactions to pets?
My husband is very allergic to cats. How does this happen? Is it random or something?
An allergic reaction is an overactive immune response to an otherwise non-pathogenic (doesn't normally cause illness) stimulus. Specifically "allergic reactions" can be classified as type 1-4 hypersensitivity reactions to allergens. An allergen is just the substance that causes the allergic response (like cat hair, pollen, etc). The hypersensitivity reaction that most people are familiar with and refer to as having "allergies" is the type 1 hypersensitivity reaction which is mediated by histamine. With this reaction, when the body is expose to the allergen (cat hair in this instance) an antibody that the body has made after previous exposure to the same allergen called IgE binds to the allergen. This then triggers a series of events where immune cells called mast cells and basophils (both of which contain histamine) to degranulate and release their histamine. Histamine is the mediator that causes the swelling/redness (hives), itching, watery eyes, runny nose, etc. This immune response to allergens is completely normal with the exception that in people with allergies for some reason it is hyperactive (hence "hypersensitivity" reactions). There are many different treatment modalities that try to dampen the overactive immune response by either blocking the cascade that leads to histamine release, or desensitizing the host to the allergen in the first place (immunotherapy). The best place for your husband to start is with a consultation to an allergist who will take a thorough exam, but likely order (or perform) allergy testing. Pending on the results, they will be able to use different therapies to help your husband with his allergies. Best of luck.