When the body detects an intruder, it launches a biological response to try to remove it.
The attacker could be a foreign body, such as a thorn, an irritant, or a pathogen. Pathogens include bacteria, viruses, and other organisms, which cause infections.
Sometimes, the body mistakenly perceives its own cells or tissues as harmful. This reaction can lead to autoimmune diseases, such as type 1 diabetes.