Final answer:
Basophils are the least abundant white blood cells that release histamines, a vasodilator, and heparin. They play a crucial role in inflammatory responses and preventing blood clotting.
Step-by-step explanation:
The type of white blood cell that releases a vasodilator and is the least abundant is the basophil. Basophils, as part of the immune system, secrete chemicals including histamines that are responsible for dilating blood vessels and increasing their permeability during inflammatory responses. They also release heparin, which inhibits blood clotting and enhances the movement of leukocytes into areas of infection. Basophils are characterized by their large granules that take up a dark blue stain, which sometimes conceals their bilobed nucleus. They typically make up less than one percent of the total white blood cell count and measure around 8-10 μm in diameter.
Answer: 5) Basophil