Antigen-antibody binding is a crucial process in the immune response, where antibodies (also known as immunoglobulins) bind to specific antigens (foreign molecules or pathogens) to neutralize or eliminate them from the body. This binding is mediated by various non-covalent interactions between the antigen and antibody molecules.

The options given are:
a) Hydrogen bond
b) Ionic interactions
c) Covalent bond
d) Hydrophobic interactions

Covalent bond does NOT form the basis of an antigen-antibody binding. Here's an explanation of each option:

a) Hydrogen bond:
- Hydrogen bonds are formed when a hydrogen atom is attracted to an electronegative atom (such as oxygen or nitrogen) in another molecule.
- In antigen-antibody binding, hydrogen bonds play a crucial role in stabilizing the interaction between specific amino acid residues in the antibody's variable region and the antigen's epitope (the specific region recognized by the antibody).
- These hydrogen bonds contribute to the specificity and strength of the antigen-antibody binding.

b) Ionic interactions:
- Ionic interactions occur between positively and negatively charged atoms or groups in molecules.
- In antigen-antibody binding, ionic interactions can occur between charged amino acid residues in the antibody's variable region and the antigen's epitope.
- These interactions also contribute to the specificity and stability of the binding.

c) Covalent bond:
- Covalent bonds involve the sharing of electrons between atoms, resulting in the formation of strong chemical bonds.
- However, covalent bonds are not typically involved in antigen-antibody binding.
- Covalent bonds would create a permanent linkage between the antigen and antibody, which is not desirable for the immune response, as the complex needs to be reversible to facilitate antigen elimination.

d) Hydrophobic interactions:
- Hydrophobic interactions occur between nonpolar (hydrophobic) molecules in an aqueous environment.
- In antigen-antibody binding, hydrophobic interactions can occur between hydrophobic amino acid residues in the antibody's variable region and hydrophobic patches on the antigen's surface.
- These interactions contribute to the stability of the binding, particularly in cases where the antigen has hydrophobic regions.

In summary, covalent bonds do not form the basis of antigen-antibody binding, while hydrogen bonds, ionic interactions, and hydrophobic interactions are all important for the specificity and stability of the binding.