The K_d is used as a measure of the affinity of an interaction. Tight interactions, also called high-affinity interactions, have a low K_d value. 

The K_d is often used to compare affinities because it intuitively gives an idea of the concentration needed to reach saturation of the interaction: it describes the concentration of ligand at which half of the target molecules are bound to ligand molecules. In this regard, the K_d is similar to the EC₅₀, but the two have important differences.