However, previous papers in this field did not discuss the polyvalency of phage particles and treated phage-ligand complex systems as 1:1 stoichiometry complexes, while this work extends the mathematical model also for non-monovalent receptors. Antibodies are not monovalent because IgG and IgA immunoglobulins contain two active sites (antigen-binding sites), whereas IgM contains ten active sites exposed on the pentamer of dimers. Their ability to bind antigens is called avidity in opposite to affinity, which refers to the interactions between one antigen and one antigen-binding site. Omitting polyvalency of the receptor may lead to mistakes in the determination of the dissociation constant . The formation of non-covalent immunological (and immunological-like) complexes between peptides and proteins may be discussed as a reversible reaction, and a qualitative description could be presented based on chemical equilibrium laws . To the best of our knowledge, the mathematical model presented here is the first which enables the calculation of Kd (affinity) from ELISA under the assumption of phage poly-valency.