Null cells constitute what percentage of peripheral lymphocytes?
**Core Concept**
Null cells are a subset of lymphocytes that lack surface receptors for immunoglobulins, complement, or T-cell markers. They are also known as CD3-CD16-CD56+ cells and are primarily involved in cytotoxic functions against infected cells or tumor cells.
**Why the Correct Answer is Right**
Null cells constitute a small percentage of peripheral lymphocytes and play a crucial role in the innate immune response. They express CD56 and CD16 receptors, which enable them to recognize and bind to target cells. The cytotoxic granules within null cells contain perforin and granzymes, which facilitate cell lysis and apoptosis. Null cells are essential for eliminating infected cells or tumor cells from the body.
**Why Each Wrong Option is Incorrect**
**Option A:** 2% - Null cells are actually a smaller subset of lymphocytes, not 2%.
**Option B:** 5% - Null cells are not as prevalent as 5% of peripheral lymphocytes, but rather a small minority.
**Option C:** 10% - Null cells are not a significant component of the peripheral lymphocyte population, making this option incorrect.
**Clinical Pearl / High-Yield Fact**
Null cells can be increased in certain conditions, such as chronic viral infections or autoimmune disorders, where the body's immune response is heightened. This can lead to an expansion of null cells, which can be an important diagnostic clue in clinical practice.
**Correct Answer: C. 1%**