Medicine & USMLE


  1. Innate Immunity
  2. Adaptive Immunity
  3. Macrophages
  4. Neutrophils
  5. Dendritic Cells
  6. Mast Cells
  7. Eosinophils
  8. Basophils
  9. Natural Killer Cells
  10. Antigens
  11. MHC I and II
  12. Antibodies
  13. B Lymphocytes Overview
  14. B Cell Stages
  15. B-Cell Activation
  16. Plasma B-Cells
  17. Memory B-Cells
  18. T-Lymphocytes Overview
  19. T-Cell Stages
  20. Types of Activated T-Cells


Antigens are any substance that induces an immune response. Specifically, antigens are usually small bits of proteins and sugar that stimulate a response by the adaptive immune system.  Antigens are often presented on the surfaces of cells by MHC, or major histocompatibility complexes, which allow the antigens to be inspected by other immune cells, like B and T-cells.

The response to an antigen depends on whether the antigen is foreign or self-made.  Foreign antigens, usually representing infection, are recognized and bound by either antibodies, which come from B-cells, or receptors, which are on T-cells. The binding of these foreign antigens induces an immune response to destroy the source of the antigen and thereby clear infections. In contrast, our bodies are usually tolerant to self-antigens, but when they’re not, we call the resulting self-damage that results autoimmunity.

Key Points

  • 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