You may have the idea from the diagram of the cell, that the specificity in the reaction between a drug its target, relies on shape-specificity, and indeed this is the case. The relationship is often described using the analogy of a lock and its key.
If we take a step back for a moment, and use the analogy of the lock
on the drugs cupboard itself we can explain how this works. To be secure
this lock must only be opened by a specific key, otherwise it would
be useless. This specificity relies on the shape of the key. There
is a close physical interaction between the lock and its key- which
must fit into the lock exactly in order to open it. Other keys, even
closely related in shape, are unable to open the lock.