Final answer:
Basophils are a type of granulocyte with a U- or S-shaped nucleus; they stain dark blue to purple with methylene blue and release histamine and heparin, resulting in allergic responses and inflammation.
Step-by-step explanation:
The cell that contains a U- or an S-shaped nucleus and granules that stain very dark, and releases histamine and heparin, is known as a basophil. Basophils are one of the types of granulocytes and they stain with basic dye methylene blue. These cells are important in the body's immune response, as they facilitate allergic responses and inflammation. The granules of basophils contain large amounts of histamine, which contributes to inflammation, and heparin, which opposes blood clotting. Basophils can be seen under a microscope and are characterized by their dark blue to purple-stained granules and typically two-lobed nucleus. High counts of basophils are associated with conditions such as allergies and parasitic infections, while low counts can occur during pregnancy, stress, or hyperthyroidism.