Intrinsic immunity

Intrinsic immunity refers to a set of recently discovered cellular-based anti-viral defense mechanisms, notably genetically encoded proteins which specifically target eukaryotic retroviruses. Unlike adaptive and innate immunity effectors, intrinsic immune proteins are usually expressed at a constant level, allowing a viral infection to be halted quickly.

Eukaryotic organisms have been exposed to viral infections for millions of years. The development of the innate and adaptive immune system reflects the evolutionary importance of fighting infection. Some viruses, however, have proven to be so deadly or refractory to conventional immune mechanisms that specific, genetically encoded cellular defense mechanisms have evolved to combat them. Intrinsic immunity comprises cellular proteins which are always active and have evolved to block infection by specific viruses or viral taxa.

The recognition of intrinsic immunity as a potent anti-viral defense mechanism is a recent discovery and is not yet discussed in most immunology courses or texts. Though the extent of protection intrinsic immunity affords is still unknown, it is possible that intrinsic immunity may eventually be considered a third branch of the traditionally bipartite immune system.

Intrinsic Immunity combines aspects of the two traditional branches of the immune system - adaptive and innate immunity – but is mechanistically distinct. Innate cellular immunity recognizes viral infection using toll-like receptors (TLRs), or pattern recognition receptors, which sense Pathogen-associated molecular patterns (PAMPs), triggering the expression of nonspecific antiviral proteins. Intrinsic immune proteins, however, are specific both in virus recognition and their mechanism of viral attenuation. Like innate immunity, however, the intrinsic immune system does not respond differently upon repeat infection by the same pathogen. Also, like adaptive immunity, intrinsic immunity is specifically tailored to a single type or class of pathogens, notably retroviruses.

...
Wikipedia