Adaptive immune system

The adaptive immune system is made of specialized cells and processes which kill pathogens or prevent their attack.
The adaptive immune system is switched on by the evolutionarily older innate immune system. This older system is non-specific, whereas the adaptive system is tailored to specific targets.
Whereas the innate immune system is found in all metazoa, the adaptive system is only found in vertebrates. It is thought to have arisen in the first jawed vertebrates.
The adaptive immune response gives the vertebrate immune system the ability to recognize and remember specific pathogens. The system mounts stronger attacks each time a particular pathogen is encountered. It is "adaptive" immunity because the body's immune system prepares itself for future challenges.
Layered defence.
The immune system protects organisms from infection with layered defences. In simple terms, physical barriers prevent pathogens such as bacteria and viruses from entering the organism.
If a pathogen breaches these barriers, the innate immune system provides an immediate, but non-specific response. Innate immune systems are found in all plants and animals.
If pathogens successfully evade the innate response, vertebrates possess a third layer of protection, the adaptive immune system, which is activated by the innate response. Here, the immune system adapts its response during an infection to improve its recognition of the pathogen.
After the pathogen has been killed, the offspring of B and T cells continue. They are ready for a faster response next time. This is a kind of 'immunological memory'. These cells allow the adaptive immune system to mount faster and stronger attacks each time this pathogen is encountered.
Both innate and adaptive immunity depend on the ability of the immune system to distinguish between self and non-self molecules. In immunology, "self" molecules are those components of an organism's body that can be distinguished from foreign substances by the immune system. Conversely, "non-self" molecules are those recognized as foreign molecules. One class of non-self molecules are called antigens (short for "anti"body "gen"erators) and are defined as substances that bind to specific immune receptors and elicit an immune response.
Functions.
Adaptive immunity is triggered in vertebrates when a pathogen evades the innate immune system and generates a threshold level of antigen.
The major functions of the adaptive immune system include:
Related pages.
For the adaptive immune system used by bacteria against viruses, see CRISPR.