Triggers of Adaptive Immunity

  1. Definition of an Antigen
    An antigen is a foreign macromolecule, typically originating from an invading microbe, that the adaptive immune system recognizes and responds to. The inherent capacity of a molecule to be recognized by the immune system is known as its antigenicity.

  2. Properties of a Good Antigen
    To effectively trigger an adaptive immune response, a molecule generally must be:

  • Large: Molecules with a mass greater than 10001000 Da are typically required for recognition; larger proteins are especially potent antigens.

  • Complex: Structural complexity (e.g., the diverse structure of proteins) enhances recognition by the immune system, whereas simple repeating polymers are often poor antigens.

  • Stable: Consistency in the molecule's structure is essential for the immune system to develop a specific response.

  • Foreign: The effectiveness of the response is influenced by how 'different' the molecule is from the host's own tissues.

Degradable: To elicit a response, a molecule must be degradable within Antigen Presenting Cells (APCs).

  1. Hapten and Carrier

  • Hapten: A small molecule (< 1000 Da) that is not immunogenic on its own but can stimulate an immune response once it is attached to a larger molecule.

  • Carrier: The larger antigenic molecule (usually a protein) to which a hapten attaches to become recognized by the immune system.

  1. Immunological Response to a Hapten-Carrier Complex
    A hapten-carrier complex forms when a small molecule (the hapten) binds to a systemic protein (the carrier). This binding modifies the carrier protein enough that the immune system no longer recognizes it as 'self' and mounts an adaptive response against the modified complex.

  • Clinically Relevant Example: Penicillin Allergy is a classic case where penicillin (the hapten) binds to host albumin (the carrier), triggering an immune response. Another example is Poison Ivy, where urushiol binds to skin proteins to cause contact dermatitis.

  1. Definition and Examples of Autoantigens
    Autoantigens are normal body components (self-antigens) that can trigger an autoimmune response if the immune system's tolerance mechanisms fail.

  • Examples: Notable autoantigens include Thyroglobulin, Myelin, and Mitochondrial Proteins.