Original: Each tip of the '' Y '' of an antibody contains a paratope ( a structure analogous to a lock ) that is specific for one particular epitope ( that is equivalent to a key ) on an antigen , allowing these two structures to bind together with precision .
Simplified: Each tip of the '' Y '' of an antibody contains a structure ( like a lock ) that fits one particular key-like structure on an antigen . This binds the two structures together .