What Is an Antigen? Definition & Immune Response Trigger

An antigen is any foreign substance that triggers an immune response. Can be:

• Proteins (bacterial/viral surface proteins).
• Polysaccharides (bacterial capsules).
• Lipids, nucleic acids, haptens (small molecules bound to carrier).

Immune Recognition

• B cells → antibodies (bind specific epitopes).
• T cells → recognize peptide-MHC complexes.
• Innate receptors (TLRs) → pattern recognition.

Antigenicity depends on foreignness, size, complexity, and degradability.