Final answer:
Basophils are the leukocytes that contain histamine in their granules, playing a key role in allergic and inflammatory reactions.
Step-by-step explanation:
The leukocyte that contains histamine in its granules is the basophil. Basophils are characterized by their large granules that pick up a dark blue stain, making them challenging to see through. These granules are filled with histamine and other pro-inflammatory compounds which are released during hypersensitive immune responses, such as allergies. Though basophils represent less than one percent of the total leukocyte count, they play a critical role in inflammatory and hypersensitivity reactions.