When is the adaptive immune system activated?

The adaptive immune system is activated when the innate immune system fails to completely eliminate a specific pathogen or threat on its own. 

When a pathogen enters the body, the innate immune system, which is the body’s first line of defense, mounts a rapid, non-specific defense. Sometimes, however, the innate immune response is unable to completely eliminate the threat. When this happens, the adaptive immune system gets triggered. 

The adaptive immune system is highly specific. It recognizes unique antigens and develops an immune response that’s tailored to the particular pathogen, enabling it to successfully and completely eliminate the threat.