How is the strength of attraction between a single antibody and antigen molecule characterized?

The strength of attraction between a single antibody and an antigen molecule is characterized by affinity. Affinity refers to the strength of the binding interaction between a single antigen-binding site on an antibody and its specific antigen. This concept indicates how tightly an antibody binds to its corresponding antigen at the molecular level. High affinity means that the antibody binds strongly to the antigen, with a lower dissociation rate, while low affinity indicates weaker binding.

In contrast, avidity refers to the overall strength of binding between an antibody and an antigen with multiple epitopes, taking into account both the individual affinities of each binding site and their simultaneous interactions. Reactivity refers to the ability of an antibody to recognize and bind to an antigen, but does not specifically measure binding strength. Valency pertains to the number of antigen-binding sites on an antibody molecule, which influences the overall binding capability but not directly the strength of interaction for a single site.

Thus, the term that best characterizes the strength of attraction between a single antibody and antigen molecule is affinity.