免疫Antigen_Recognition_by_B-cell_and_T-cell_Receptors.pptVIP

* 9 * 8 * * * 16 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 7 Polio virus epitopes shown in white VP1 protein The basis of antibody binding Depending on the nature of the antigen: Hydrophobic interactions Van der Waals forces Electrostatic interactions Hydrogen bonds Non-covalent, therefore reversible, binding Equilibrium affinity Ka= [Ag:Ab] [Agfree][Abfree] Affinity(亲和力) and Avidity(亲合力) Affinity: the strength of binding between a single binding site and a single ligand(epitope) Avidity: the strength of binding between a molecule and a complex ligand, e.g. if there are multiple binding sites then the avidity may be increased by increasing the number of binding sites or by increasing the affinity of those binding sites Affinity and Avidity II IgM is produced early in an immune response when the affinity for antigen often is low; as an immune response continues, antibody affinity is improved, this is combined by “class switching” to the use of smaller molecules (IgG, IgE and IgA). The increased affinity compensates for the decrease in number of binding sites in maintaining the overall avidity for antigen Clonal Selection Theory pre-existence of of many different potential antibody producing cells each cell displays surface receptors for specific antigens antigen encounter selects cells Postulates of the Clonal Selection Hypothesis Each lymphocyte bears a single type of receptor of a unique specificity Interaction between a foreign molecule and a lymphocyte receptor capable of binding that molecule with high affinity leads to lymphocyte activation The differentiated effector cells derived from an activated lymphocyte will bear receptors of identical specificity to those of the parental cell from which that lymphocyte was derived Lymphocytes bearing receptors specific for self molecules are deleted at an early stage in lymphocyte development and are therefore absent from the repertoire 1 2 13