Acquired (Adaptive) Immunity

ii. Acquired immunity : Acquired immunity (also called adaptive or specific immunity) is the resistance an individual builds up over the course of their life, through the formation of antibodies that neutralise specific antigens. Unlike innate immunity, it has several defining features: specificity (a specific antibody or T-lymphocyte is made against a particular antigen/pathogen), diversity (it can recognise an enormous range of different pathogens), self/non-self discrimination, and — most distinctively — memory: the first time the immune system meets a given antigen (the 'first encounter'), it mounts a response and eliminates the invader, but it also retains a memory of that encounter, so a second exposure to the same pathogen triggers a faster and stronger response.

Types of Acquired Immunity : Acquired immunity is of two kinds:

• Active immunity is the resistance an individual develops as a result of their own antigenic stimulus, and it can be natural or artificial. Natural active immunity develops after a person is actually infected by a pathogen and recovers — for example, someone who has recovered from measles develops lifelong natural active immunity to it. Artificial active immunity is induced by vaccines: a vaccine is introduced into the body to stimulate the immune system into producing antibodies itself (e.g. polio or BCG vaccine), and such immunity may be temporary or permanent. In both cases, because the body's own cells actively make the antibodies, this kind of immunity tends to be long-lasting. …