Acquired immunity

Unlike innate immunity, acquired immunity develops after birth and is targeted at a specific pathogen the body has encountered. Its hallmark features are antigenic specificity (it responds to a particular antigen), diversity (an enormous range of possible antigens can be recognised), the ability to tell self from non-self, and — most distinctively — immunological memory, which allows a faster, stronger response on re-exposure.

By mechanism, acquired immunity has two components. In cell-mediated immunity (CMI), pathogens are destroyed directly by cells — T cells, macrophages and natural killer cells — without antibody production. In antibody-mediated (humoral) immunity, B cells, assisted by antigen-presenting cells and T-helper cells, manufacture antibodies that neutralise the pathogen; antibody production is a feature unique to vertebrates. …