## **Core Concept**
The CD4 count, also known as the T helper cell count, is a critical measure of the immune system's health, particularly in assessing the progression of HIV infection. CD4+ T cells play a central role in cell-mediated immunity. A normal CD4 count in a healthy adult typically ranges from 500 to 1600 cells per microliter (μL) of blood.

## **Why the Correct Answer is Right**
The correct answer, , indicates a CD4 count of 500-1600 cells/μL, which is considered the normal range for a healthy adult. This range is crucial for the proper functioning of the immune system. CD4+ T cells help activate and direct other immune cells, such as B cells to produce antibodies and cytotoxic T cells to kill infected cells or produce chemical signals that activate the immune response.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This range is lower than the normal range and might be seen in individuals with immune compromise, including those with HIV/AIDS or other immunodeficiency conditions.
- **Option B:** This is significantly higher than the upper limit of the normal range and is not typically seen in healthy adults.
- **Option D:** This range includes values that are below the normal range and could indicate immune system dysfunction.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that a CD4 count below 200 cells/μL is a criterion for diagnosing AIDS in individuals with HIV infection. Monitoring CD4 counts is essential for managing HIV infection and determining the risk of opportunistic infections.

## **Correct Answer:** . 500-1600 cells/μL