If a pathogen makes its way past innate defenses, organisms rely upon knowledge-based adaptive immune systems. Even microbes have adaptive immunity, though their systems are distinct from those evolved by the earliest jawed vertebrates, which have been carried on in later-evolved vertebrates as a highly conserved trait.
The terms acquired immunity and adaptive immunity are used interchangeably. In vertebrates, adaptive response is activated by the innate immune system.
Living a short life can be advantageous if you are pathogen – but only if you produce offspring.
Innumerable pathogens evolved strategies to evade the innate immune system. Disguises and subterfuges are picked up from surrounding cellular material and incorporated into the next generation. Many micro-marauders can shape their exteriors to completely avoid complement activation. These are not random acts by lucky microbes. This is intelligence at work in a life form with no recognized brain, doing what parents everywhere do: try to make a better life for their offspring.
An antibody is a protein that remembers past encounters with pathogens. The remembered molecular pattern is an antigen, short for antibody generator.
An antigen is a memory molecule which can provoke an immune system response, except in the case of self-antigens. A self-antigen is a molecule of cellular self-recognition.
An antibody recognizes a pathogen upon contact, without mistakenly targeting the body’s own cells or friendly microbes. It attaches itself to the ne’er-do-well and calls for reinforcements, including the complement system and phagocytes.
Specialized lymphocytes – B & T cells – are the front-line warriors of the adaptive immune system. Some of the lymphocytes that win the war against an infection transform into historians. These memory cells pass on the tale of past battles so that an immune response can be faster and more effective the next time.