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Cross-reactivity, in a general sense, is the reactivity of an observed agent which initiates reactions outside the main reaction expected. This has implications for any kind of test or assay, including Medical test in medicine, and can be a cause of false positives. In immunology, the definition of cross-reactivity refers specifically to the reaction of the immune system to . There can be cross-reactivity between the immune system and the antigens of two different , or between one pathogen and proteins on non-pathogens, which in some cases can be the cause of Allergy.
In drug screening, because many urine drug screens use immunoassays there is a certain amount of cross-reactivity. Certain drugs or other chemicals can give a false positive for another category of drug.
An adaptive immune response is specific to the antigen that stimulated it (called the immunogen). However, many naturally occurring apparent antigens are actually a mixture of (for example, from , , , or pollen) comprising several . Contact with a complex antigen such as a virus will stimulate multiple Immune system to the virus' different macromolecules as well as the individual epitopes of each macromolecule. For example, the tetanus toxin is a single protein macromolecular antigen but will stimulate many immune responses due to the tertiary structure of the protein yielding many different epitopes. The toxin that creates the immune response will have an epitope on it that stimulates the response. Denaturing the protein may 'disarm' its function but allow the immune system to have an immune response thus creating an immunity without harming the patient.
Cross reactivity has implications for flu vaccination because of the large number of strains of flu, as antigens produced in response to one strain may confer protection to different strains. Cross-reactivity need not be between closely related viruses, however; for example, there is cross-reactivity between Orthomyxoviridae-specific CD8+ and hepatitis C virus antigens.
Cross-reactivity may be caused by identical carbohydrate structures on unrelated proteins from the same or different species. Such cross-reactive carbohydrate determinants (CCDs) are an issue in allergy diagnosis, where about a fifth of all patients displays IgE antibodies against Asn-linked oligosaccharides (N-glycans) containing core α1,3-linked fucose. As CCDs apparently do not elicit allergic symptoms, a positive in vitro test based on IgE binding to CCDs must be rated as false positive.
Although allergic reactions typically require prior sensitization to a specific allergen, clinical symptoms can sometimes occur upon first exposure to a food or substance; this is explained by IgE cross-reactivity, where prior sensitization to structurally homologous proteins from other sources leads the immune system to recognize similar proteins in the new allergen as triggers, even though the affected individual has never previously consumed or contacted it.
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