**Core Concept:** Neuroepithelial receptors are a type of secondary receptors found in the nervous system. These receptors are activated by specific neurotransmitters, leading to a cellular response. Secondary receptors are distinguished from primary receptors (e.g., ion channels) as they involve intracellular signaling pathways.

**Why the Correct Answer is Right:** The correct answer is "D" because neuroepithelial receptors are not present in the endocrine system. Endocrine cells, unlike neurons, do not have specialized receptors to respond to neurotransmitters directly. Instead, endocrine cells release hormones into the bloodstream, which then bind to target cells or tissues to initiate a response.

**Why Each Wrong Option is Incorrect:**

A. **Neuroepithelial receptors are present in the immune system.** In the immune system, specialized cells such as lymphocytes and macrophages play a crucial role in immune responses and do not rely on neurotransmitters for communication.

B. **Neuroepithelial receptors are not found in the circulatory system.** The circulatory system primarily involves blood vessels, blood cells, and the heart, which do not possess neuroepithelial receptors. Instead, the cardiovascular system relies on neurotransmitters and hormones to regulate its functions.

C. **Neuroepithelial receptors are not found in the respiratory system.** The respiratory system primarily focuses on gas exchange and does not involve direct communication between neurons and respiratory system cells. Neuroepithelial receptors are not relevant in this context.

**Clinical Pearl:** Understanding the presence and absence of neuroepithelial receptors in different organ systems helps differentiate between various physiological processes and mechanisms. Differentiating between primary and secondary receptors is essential for understanding cellular responses and communication within the human body.