An example of a protein receptor given in the material is

• A. hair
• B. enzymes
• C. antibodies
• D. collagen

Answer: (A)

Protein receptors are proteins that receive signals by binding specific molecules (ligands) and then trigger a cellular response. An example often discussed is antibodies acting as receptors in the immune system; the membrane-bound form of an antibody on B cells (the B cell receptor) binds a particular antigen and starts a signaling cascade that activates the cell. In contrast, hair and collagen are structural proteins, and enzymes are proteins that catalyze reactions rather than serve primarily as signal receivers. If you’ve seen hair mentioned as a receptor in some materials, that typically refers to sensory contexts where nerve endings associated with hair detect stimuli—it’s the nerve-ending receptors, not the hair shaft itself, that function as the actual receptors.