Cross-reactivity

Cross-reactivity applies to the reaction between two different species as opposed to the self-reactivity. In chemistry it means a reaction between two different molecules. At the same time each of these molecules is able to react with the identical molecule, or as it is usually described, to react with itself. In immunology, the cross-reactivity has a more narrow meaning of the reaction between an antibody and an antigen that differs from the immunogen. It is sometimes also referred to as crossimmunity or cross-protective immunity, although cross-reactivity does not necessarily infer cross-protection. A few examples of cross-reactivity have been confirmed in humans, one of which involves influenza virus-specific CD8+ T cell and hepatitis C virus antigens.

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 macromolecules (for example, from pathogens, toxins, proteins, or pollen) comprising several epitopes. Contact with a complex antigen such as a virus will stimulate multiple immune responses 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.

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