Final answer:
Proteins are prevented from crossing the blood-brain barrier (BBB). The BBB is a physiological barrier that restricts the passage of certain molecules into the central nervous system (CNS). It protects the CNS from exposure to harmful substances.
Step-by-step explanation:
The substance that is prevented from crossing the blood-brain barrier (BBB) is proteins. The BBB is a physiological barrier that restricts the passage of certain molecules from the circulating blood into the central nervous system (CNS). While glucose, water, and some small particles can enter the CNS, proteins cannot. This barrier protects the CNS from exposure to toxic or pathogenic substances, but it also makes it harder to develop pharmaceuticals that can affect the nervous system.