the various cells and tissues in humans and other vertebrate animals which enable them to mount a specific protective respondse to invading micro-organisms, parasites, etc and, protecting the body from infection and often setting up a long-lasting immunity to reinfection
The immune system is shaped by evolution to react to foreign infectious agents such as parasites, bacteria and viruses, but not to ‘self”. To eradicate infection, it employs a battery of mechanisms, from relatively non-specific defences at skin and mucosal barriers to circulating antibody proteins and immune cells that are highly specific for the infection. It retains a ‘memory’ for the infection, so that on a subsequent encounter it is primed and ready to respond immediately – Professor Leonard C Harrison