The immune system uses a two-layer recognition system:
**1. Innate immunity (fast, non-specific):** Pattern Recognition Receptors (PRRs) — especially Toll-like receptors on immune cells — detect conserved molecular patterns called PAMPs (Pathogen-Associated Molecular Patterns) shared by many pathogens (e.g., bacterial cell wall components, viral RNA). This triggers immediate inflammation.
**2. Adaptive immunity (slow, specific):** Antigens (unique surface proteins) from the pathogen are presented by dendritic cells to T and B lymphocytes via MHC molecules. B cells produce antibodies that bind specifically to those antigens. Memory cells are then retained for faster future responses — this is the basis of vaccines.