Which option is a mechanism of desensitization?

• A. Increased receptor synthesis
• B. Receptor internalization
• C. Increased ligand affinity
• D. Prolonged receptor activation

Answer: (B)

Desensitization happens when the cell dampens its response to a persistent signal by reducing receptor signaling at the surface. Receptor internalization is the key mechanism: after receptor activation, the receptors are pulled into the cell via endocytosis, lowering the number of receptors available on the surface to bind ligand. With fewer receptors, the same amount of ligand elicits a smaller response, which is the essence of desensitization. This process often involves receptor phosphorylation and arrestin binding, which uncouples the receptor from its signaling machinery and promotes internalization. The effect is typically reversible as receptors can be recycled back to the surface or degraded, allowing resensitization later.

In contrast, increasing receptor synthesis would boost responsiveness, increasing ligand affinity would heighten sensitivity rather than desensitize, and prolonged receptor activation maintains signaling rather than reducing it.