What is an antigen?

An antigen is any substance usually a protein on a pathogen or foreign particle that triggers an immune response.

• Causes the body to produce antibodies shaped to bind to and neutralise it.
• "Antigen" = "antibody generator"
• Lock and key analogy: each antigen has a unique structure; antibodies match it perfectly.
• Vaccines introduce antigens in a safe form to prime the immune system.

Common mistake: Antigen = the trigger; antibody = the response.

Summary: An antigen is a foreign substance that triggers antibody production the basis of immune defense and vaccination.