One term that is used is antigen. An antiGEN will GENerate an ANTIbody which will 'kill' the antigen.
antigen- any substance the body regards as foreign
antigen
If it can be harmful, it can be referred to as pathogen.
A foreign substance is a substance that doesn't belong somewhere. A piece of glass or splinter is a foreign substance that people get in their body.
A foreign substance is a substance that doesn't belong somewhere. A piece of glass or splinter is a foreign substance that people get in their body.
antigen
lymphokine
pathogen
The body usually forms antibodies in response to invasion by a foreign material.
The body can reject implants and, of course, blood if it's not your own type or O negative (I think). You can look at allergies the same way - hay fever, nuts, strawberries - the body detects them as harmful even though they are harmless.
Antigenan·ti·gen/ˈantijən/Noun:A toxin or other foreign substance that induces an immune response in the body, esp. the production of antibodies.
When a foreign substance enters the body.