2.5.2 Antigens

Key Features of Antigens:

1. Definition:

• An antigen is a molecule, usually a protein or glycoprotein, found on the surface of cells, viruses, or other particles.

2. Types of Antigens:

• Self-antigens:
• Found on the surface of the body's own cells.
• Identified as "normal" by the immune system, preventing an immune response.
• Non-self antigens:
• Found on pathogens, abnormal cells, or foreign substances.
• Trigger an immune response.

3. Sources of Non-Self Antigens:

• Pathogens (e.g., bacteria, viruses, fungi): Have unique antigens on their surface.
• Toxins: Produced by pathogens and act as antigens.
• Cells from other individuals: Transplanted tissues and blood from different individuals may have different antigens, leading to rejection.
• Cancerous or abnormal cells: Display abnormal antigens that are recognised as non-self.

Role of Antigens in the Immune Response:

4. Detection:

• Lymphocytes (B cells and T cells) patrol the body, identifying antigens on the surface of cells.

5. Activation:

• When a non-self antigen is detected:
• B lymphocytes produce antibodies specific to the antigen.
• T lymphocytes initiate the destruction of infected or abnormal cells.

6. Clonal Selection:

• The immune system selects specific lymphocytes that match the antigen.
• These lymphocytes divide rapidly to form a clonal population, enabling a targeted response.

Examples of Antigen Interaction:

7. Vaccination:

• Vaccines introduce harmless forms of antigens into the body, stimulating the production of antibodies and memory cells without causing disease.

8. Blood Transfusions:

• Blood type compatibility depends on antigens (e.g., A, B, Rh) present on red blood cells.
• Mismatched blood types can trigger an immune response.

9. Transplant Rejection:

• Foreign antigens on transplanted organs can be recognised as non-self, leading to immune rejection unless suppressed by medication.