• Antigens
    • Any substance (usually protein or sugar) that stimulates an adaptive immune response (e.g. antibody production)
    • Includes:
      • Foreign antigens (e.g. bacteria, viruses, fungi, parasites)
        • Generates immune response by B and T cells against antigen
      • Self-antigens (from within our own body)
        • Adaptive immune system is self-tolerant (no immune response)
        • Immune responses to self-antigens causes autoimmunity
    • Presented on MHC by antigen presenting cells
      • External/exogenous antigens are phagocytosed and presented on MHC2, while internal/intracellular antigens are directly presented on MHC1