why system of our body is activated in response to in fection ? how it responds

Immune system is activated in response to the infection.The function of immune system is to protect the organism from diseases. Foreign substances are recognised and eliminated by our body's immune system. In order to fight against these diseases and foreign materials, our immune system triggers off both general and local response. Local response at the site of infection is initiated to prevent the spread of infection to neighbouring sites, while general response is generated to locate any infectious particles present elsewhere in the body and remove them.