A 56-year-old man is brought to the ED 45 minutes after sudden onset of right arm weakness and difficulty speaking. He has a history of atrial fibrillation and is not taking anticoagulation. Noncontrast CT of the head shows no hemorrhage. He is given IV alteplase. Later, the team discusses how circulating inflammatory mediators and toxins are normally restricted from entering the brain parenchyma. Which of the following cellular structures is a key component of this protective barrier?

(Assume normal adult neuroanatomy.)