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Cross-reactivity is the ability of an immune cell to attack a foreign cell that's different from the one that created it. Immune cells are made by the body to destroy disease-causing substances. Each immune cell attacks a certain type of invasive agent. If a different type of disease-causing substance has similar chemical properties to a previously encountered invasive agent, then cross-reactivity can occur allowing the immune cell to attack the new invader. This process is also known as cross-immunity and cross-protective immunity.
A pathogen is a disease-causing agent such as a virus, bacteria, parasite, or fungus. When a pathogen enters the body, its presence triggers the innate immune system. The response of the innate immune system is general, but often sufficient to fight off most pathogens. If the response of the innate immune system isn't enough to fend off the body's invaders, the adaptive immune system is triggered, and it mounts a specified attack.
All jawed vertebrates, including humans, have an adaptive immune system. It differs from the simpler innate immune system in that its response to a pathogen is very specific. The adaptive immune system is able to recognize proteins, or antigens, on the surface of the pathogenic cells and create an immune cell, or antibody, specifically designed to destroy it.
Sometimes a different pathogen will have proteins that are the same or similar to the ones on a pathogen the body has already encountered. The adaptive immune system recognizes the antigen and uses already created antibodies to attack it, thereby destroying the new pathogen. This process is called cross-reactivity.
The term cross-reactivity also applies to allergy suffers. The immune system goes through the same process, however, the antigen causing it isn't pathogenic, but the body perceives it to be a potentially dangerous or disease-causing threat. In this scenario the antigen is called an allergen.
Any substance that causes an allergic reaction is an allergen. Allergic reactions vary highly in both severity and presentation. Allergy sufferers who are allergic to the same substance can produce different reactions to it. For example, one person who is allergic to grass will suffer from nasal congestion, while another will get a skin rash. Allergic reactions are the side effects of the immune system attacking an allergen.
Allergen-induced cross-reactivity can occur when the body is exposed to similar or closely related allergens. For example, people who are allergic to birch pollen should avoid eating raw apples because they have similar chemical compounds. The immune cells recognize a similarity in the chemical composition of the nonallergenic substance and attack it, causing a cross-reactive allergy.