The characteristic of a substance that can provoke an immune response (see IMMUNITY). This includes how ‘foreign’ a substance entering or contacting the body is; route of entry; dose; number and period of exposure to antigen; and genetic make-up of the host. The characteristics of molecules that determine immunogenicity are:

• Foreignness: molecules recognised as ‘self’ are generally not immunogenic; the body tolerates these self-molecules. To be immunogenic, molecules must be recognised as non-self or foreign.

• Molecular size: proteins with high molecular weights (over 100,000) are the most effective immunogens; those below 10,000 are weakly immunogenic; and small ones, for example, AMINO ACIDS, are non-immunogenic.

• Chemical complexity: the greater the chemical complexity, the more immunogenic the substance.

• Dosage, route and timing of antigen administration: all these are important factors.