The lock and key model is a historical analogy used in biology to describe the mechanism of molecular recognition, proposing that binding interactions occur when two molecules—a receptor and a ligand, or an enzyme and its substrate—possess geometrically complementary sites. This model suggests that the binding site on the receptor is rigid, possessing a highly specific shape that only allows for the binding of a molecule with a perfectly complementary structure, much like a specific key fitting into a specific lock. While modern understanding has refined this concept to include induced fit, the model remains foundational for explaining the high specificity required for biological processes, such as the action of enzymes, immune detection, and signal transduction.