| Feature | Self Antigens | Non-Self Antigens |
|---|---|---|
| Origin | Produced by the organism's own cells | Found on foreign cells, viruses, or toxins |
| Immune Response | Do not trigger an immune response | Stimulate an active immune response |
| Function | Identify 'friend' cells to prevent autoimmunity | Identify 'foe' cells for destruction |
| Examples | MHC markers on human leukocytes | Viral capsids, bacterial walls, pollen |
Antigenic variability refers to the ability of certain pathogens, particularly viruses like influenza, to frequently change their surface antigens through genetic mutations.
When the genes coding for surface proteins mutate, the resulting antigens have a different tertiary structure. This means that memory cells produced during a previous infection will no longer have receptors complementary to the new antigen shape.
This mechanism allows pathogens to evade the host's secondary immune response. Consequently, the host must undergo a new primary immune response, leading to repeated illness from the 'same' disease.
Precision in Definitions: When asked to define an antigen in the context of an immune response, always specify that it is a non-self or foreign macromolecule that stimulates an immune response.
Link Structure to Function: Always explain that the specificity of the immune response is due to the complementary shape between the antigen and the lymphocyte receptor.
Application of Variability: Be prepared to use the concept of antigenic variability to explain why vaccines (like the flu jab) must be updated annually. The vaccine must contain the antigens of the most current circulating strains.
Common Error: Do not confuse 'antigen' with 'pathogen'. An antigen is a specific molecule on the pathogen, not the whole organism itself.
