Final answer:
White blood cells can cross the blood-brain barrier in response to specific signaling related to tissue damage, contributing to the BBB's function. The BBB's tight junctions usually prevent passage of substances, but can be opened temporarily for immune responses.
Step-by-step explanation:
The cells that can pass through the blood-brain barrier (BBB) by signaling to open up the junctions are white blood cells (leukocytes). Despite the BBB's role in protecting the central nervous system (CNS) from exposure to potentially harmful substances, white blood cells can cross it in response to signaling, reacting to tissue damage, and contributing to the blood-brain barrier. The capillary endothelial cells in the brain form tight junctions which typically prevent almost all substances from directly moving into the CNS. However, in response to certain signals, such as those related to inflammation or tissue damage, these junctions can be temporarily modified to allow immune cells to penetrate the BBB and perform their protective functions.
Aside from leukocytes, primarily nutrients such as glucose and amino acids can pass through the BBB through active transport mechanisms.