What factors affect the binding between antigen and antibody in ag-ab reaction?

Temperature, ionic strength, and pH can influence affinity constants of antigen-antibody reactions. 

• The ideal temperature for the antigen-antibody reaction varies depending on the chemical characteristics of the epitope and paratope, as well as the types of bonds involved in their interaction. For instance, hydrogen bonds tend to be more stable at lower temperatures. This is significant particularly when dealing with carbohydrate antigens, as these bonds are crucial for their recognition and binding by antibodies. 
• The impact of ionic strength on antigen-antibody reactions is crucial in blood group serology. Chloride and sodium ions (present in saline) can affect the reaction by clustering around antigen-antibody complexes and partially neutralizing charges. This interference causes difficulties with low-affinity antibodies.
• The pH level affects the equilibrium constant of the antigen-antibody complex most notably within the 6.5-8.4 range. Outside of this range the reaction is significantly inhibited, with pH levels below 6.5 or above 8.4 resulting in a 100-fold decrease in the equilibrium constant compared to pH 6.5 to 7.0. Extreme pH conditions can induce conformational changes in antibodies, disrupting their ability to bind with the antigen.
• Various types of molecular interactions, including hydrogen bonding, hydrophobic interactions and van der Waals forces, play roles in the binding process. The specific amino acid side chains found in the hypervariable loops of antibodies usually interact directly with the antigen. These interactions affect how accurately the antibody can recognize the antigen and how strongly it binds to it.
• The precise interaction between the epitope and paratope on the immunoglobulin molecule involves highly localized regions of the molecules. This specific binding event requires overcoming an inherent repulsion between the two molecules. When the epitope and paratope molecules come into close proximity, van der Waals forces become significant. Hydrogen bonds form between hydrogen atoms and electronegative atoms (such as oxygen or nitrogen) on both the antigen and antibody molecules. When hydrophobic regions on the antigen and antibody come into contact, they can form hydrophobic interactions, contributing to the stability of the complex.