Antigen and Antibodies

Immunology Overview

  • Focus on antigens and antibodies as foundational concepts in immunology.

Key Terminology

  • Immunogen:

    • Definition: A macromolecule that triggers an adaptive immune response and reacts with antibodies produced.

    • Often used interchangeably with the term antigen.

  • Antigen:

    • Definition: Any substance that can be specifically bound by an antibody or a T lymphocyte receptor.

  • Haptenins:

    • Definition: Lower molecular weight molecules that can bind to antibodies, but require a macromolecule carrier to stimulate an immune response.

    • Distinction: Antigens can induce a response independently; haptens need assistance from a carrier.

Structure of Antigens

  • Epitope:

    • Definition: The specific part of the antigen that reacts with antibodies or T cell receptors.

    • Visual Representation: Illustrated as a highlighted region on the antigen.

Types of Antigens

  • Self Antigens:

    • Definition: Antigens that are normal components of our own cells.

    • Example: Histocompatibility Antigens (MHC)

    • Major Histocompatibility Complex (MHC):

      • Present on the surface of nucleated cells, including white blood cells.

      • Recognized as self by the immune system, hence no reaction occurs.

      • Importance: Critical in transplantation to avoid rejection when introducing foreign cells.

  • MHC Antigens:

    • Two Types:

    • MHC Class I:

      • Present on all nucleated cells.

    • MHC Class II:

      • Primarily found on professional antigen-presenting cells (APCs), facilitating helper T cell interactions.

    • Practical Implications: MHC molecules are essential in transplant compatibility, forensic testing, and disease association.

  • Autoantigens:

    • Definition: Self antigens that can be misidentified as foreign, potentially leading to autoimmunity.

    • Result: The body may create autoantibodies against its own antigens, which can be damaging.

Blood Group Antigens

  • Representation on Red Blood Cells:

    • Blood Groups defined by presence of specific antigens on their surface.

    • A antigen (in purple)

    • B antigen (in blue)

    • Rh/D antigen (in yellow)

  • Blood Type Examples:

    • A Positive: A antigen + Rh antigen

    • B Negative: B antigen only

    • O Negative: No A/B antigens, no Rh presence

  • Antibody Presence:

    • O type blood can have antibodies against A, B, and Rh.

    • B type blood can have antibodies against Rh and A.

    • A type blood can have antibodies against B.

  • Requirements in Blood Banking:

    • Further discussion of blood types will take place in dedicated sessions.

Chemical Nature of Antigens

  • **Composition:

    • Predominantly Proteins:**

    • Proteins are excellent antigens due to high molecular weight and structural complexity, encompassing primary, secondary, tertiary, and quaternary structures.

    • Lipids:

    • Less commonly antigenic because of their simpler structures.

Characteristics of Effective Antigens

  • Key Features:

    • Foreignness:

    • Antigens must be recognized as foreign to elicit an immune response.

    • Route of Exposure:

    • How the antigen enters the body influences its immunogenicity.

    • Dose:

    • Higher quantities typically lead to stronger immune responses (e.g., contrast between 1 E. Coli vs. 10 million E. Coli).

    • Stability and Complexity:

    • Structural stability and complexity enable better antigen recognition by the immune system.

  • Conclusion:

    • Lipids are mostly excluded from being good antigens due to low structural stability and complexity.