Antigens

Summary

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
        • Recognized and bound by antibodies (free or attached to B-cells as B-cell receptors) or T-cell receptors 
    • 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