Which best describes the law of mass action?

• A. Once antigen-antibody binding takes place, it is irreversible.
• B. The equilibrium constant depends only on the forward reaction.
• C. The equilibrium constant is related to strength of antigen-antibody binding.
• D. If an antibody has a high avidity, it will dissociate from antigen easily.

Answer: (C)

The key idea is that antigen–antibody interactions follow reversible, mass-action kinetics. The rate at which complexes form is proportional to the product of the antigen and antibody concentrations, and the rate at which they dissociate is proportional to the amount of complex present. At equilibrium, these rates balance, and the equilibrium constant that describes this balance reflects how tightly the two bind. A larger equilibrium constant (or a smaller dissociation constant) indicates stronger binding between antigen and antibody. So, saying that the equilibrium constant is related to the strength of antigen–antibody binding captures what the law of mass action tells us about how tightly the interaction holds. The other statements are off because binding is not irreversible, the equilibrium constant depends on both forward and reverse rates, and high avidity denotes strong overall binding rather than easy dissociation.