explain what is meant by an antigen (see 4.1.3) and state the difference between self antigens and non-self antigens

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology – The Immune System: Antigens

The Immune System

What is an Antigen?

An antigen is any substance that can be recognised by the immune system and elicit an immune response. Antigens are typically proteins or polysaccharides on the surface of cells, viruses, bacteria, toxins, or transplanted tissues. When an antigen is detected, specific immune cells (e.g., B‑cells and T‑cells) are activated to produce antibodies or cell‑mediated responses that target the antigen.

Self Antigens vs. Non‑self Antigens

Antigens are classified according to whether they originate from the body’s own cells or from external sources.

FeatureSelf AntigensNon‑self Antigens
OriginProduced by the body’s own cells (e.g., normal proteins, blood group antigens)Derived from foreign entities (e.g., pathogens, transplanted tissue, allergens)
Immune toleranceUsually tolerated; immune system does not attack under normal conditionsRecognised as “foreign”; triggers immune response
ExamplesMajor histocompatibility complex (MHC) molecules, self‑cell surface proteinsBacterial cell wall components, viral capsid proteins, pollen proteins
Clinical relevanceAutoimmune diseases occur when tolerance fails and self antigens are attackedVaccination, infection, transplant rejection, allergic reactions

Key Points to Remember

  1. Antigens are the specific molecules that the immune system recognises.

  2. Self antigens are normal components of the body; they normally do not provoke an immune response because of immune tolerance.

  3. Non‑self antigens are foreign; they are the primary targets of protective immune responses.

  4. Breakdown of tolerance to self antigens leads to autoimmune disorders.

Suggested diagram: Illustration showing a self cell with self antigens (green) and a pathogen with non‑self antigens (red), with immune cells recognising the non‑self antigens.