Which of the following is true about histamine?

Histamine plays a significant role in the body's immune response, particularly during allergic reactions. When an allergen enters the body, immune cells called mast cells and basophils release histamine. This release leads to various physiological changes, including increased permeability of blood vessels, which allows fluid and immune cells to reach the affected tissue. This response is part of the body's effort to eliminate the allergen. The release of histamine can result in symptoms typically associated with allergies, such as swelling, redness, itching, and mucus production.

In contrast, histamine does not directly lower blood pressure; rather, it can cause blood vessel dilation, which may lead to a drop in blood pressure in some scenarios, but it's not its primary function. Furthermore, histamine is not known for reducing inflammation; instead, it can actually contribute to inflammation as part of an allergic response. Lastly, while weight gain can be influenced by a variety of factors, histamine itself does not cause weight gain. It’s primarily a mediator in allergic and inflammatory responses rather than a direct influence on weight or metabolism.