If a phage is treated as monovalent, the measured Kd does not refer to the thermodynamical strength of single peptide-TnT interactions. However, the resultant affinity (strength of single interaction) and valency (number of binding sites per particle) are known in immunology as avidity . Avidities are most significant for IgM antibodies that consist of five subunits presenting two antigen-binding domains on each one. As a result, a single IgM antibody can theoretically bind ten epitopes, causing 1:10 antibody-ligand complex stoichiometry.