Haptens

Haptens

Haptens

  • Haptens are small organic molecules that are antigenic but not immunogenic.
  • They are not immunogenic because they cannot activate helper T cells.
  • Failure of hapten to activate helper T cells is due to their inability to bind to MHC proteins; they cannot bind because they are not proteins and only proteins can be presented by MHC proteins.
  • Haptens are univalent hence cannot activate B cells by themselves.
  • The haptens, however, can activate B cells when covalently bound to a “carrier” protein.
  • When bound with a carrier molecule, they form an immunogenic hapten–carrier conjugate.

Hapten–carrier conjugate

  • In this process, the haptens combine with an IgM receptor on the B cells, and the hapten–carrier protein complex is internalized.
  • A peptide of the carrier protein is presented in association with class II MHC protein to the helper T cells.
  • The activated helper T cells then produce interleukins, which stimulate the B cells to produce antibodies to hapten.
  • Animals immunized with such a conjugate produce antibodies specific for:
    • The hapten determinant.
    • Unaltered epitopes on the carrier protein.
    • New epitopes formed by combined parts of both the hapten and carrier.
  • Hapten carrier molecule is bound to surface immunoglobulins on B cells via the hapten epitopes.
  • The hapten–carrier molecule is then taken in, processed, and pieces of the carrier are presented by these B cells and TH cells.
  • In the body, the formation of hapten–carrier conjugates is the basis for development of allergic responses to drugs, such as penicillin.

Haptens

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