**Core Concept**
Histamine is a biogenic amine involved in various physiological and pathological processes. It plays a crucial role in immune responses, allergic reactions, and the regulation of various bodily functions.

**Why the Correct Answer is Right**
Histamine is indeed found in mast cells, which are a type of immune system cell responsible for releasing histamine during an allergic reaction. When released, histamine causes blood vessels to dilate, leading to increased blood flow and permeability, and it also stimulates gastric acid secretion, which aids in digestion. Furthermore, histamine is involved in the regulation of arousal and blood pressure through its action on the brain and the autonomic nervous system. Specifically, histamine receptors in the brain are involved in regulating the sleep-wake cycle and blood pressure, with histamine causing an increase in blood pressure and arousal.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is partially incorrect because histamine is also found in other cells, such as neurons and macrophages, in addition to mast cells.
**Option B:** This statement is partially incorrect because histamine can also decrease gastric acid secretion in certain situations, such as during the healing of ulcers.
**Option C:** This statement is partially incorrect because histamine's role in arousal and blood pressure is more complex and can vary depending on the specific context and receptors involved.

**Clinical Pearl / High-Yield Fact**
Histamine's role in allergic reactions and immune responses makes it a key player in conditions such as anaphylaxis, asthma, and atopic dermatitis. Understanding the mechanisms of histamine release and its effects on the body is crucial for the diagnosis and treatment of these conditions.

**✓ Correct Answer: D. All of the above**