**Question:** Natural killer (NK) cells attacks which of the following cells MI-

A. Lymphocytes
B. Macrophages
C. Neurons
D. Bone marrow cells

**Core Concept:** Natural killer (NK) cells are a type of lymphocyte, part of the body's immune system. They play a crucial role in fighting infections and cancer by recognizing and eliminating target cells. NK cells can target and destroy cells that have lost their ability to express major histocompatibility complex (MHC) molecules, known as "missing self" recognition.

**Why the Correct Answer is Right:** Natural killer cells primarily attack cells that have lost their expression of major histocompatibility complex (MHC) molecules, specifically lymphocytes (like Option A). NK cells are part of the adaptive immune system and specifically target cells that lack MHC molecules, which are often found in viral infections or cancer cells.

**Why Each Wrong Option is Incorrect:**

B. Macrophages (Option B) are part of the innate immune system, and although they can engulf foreign particles and pathogens, they are not the primary target of NK cells. Macrophages function differently from NK cells and are not directly attacked by them.

C. Neurons (Option C) are part of the nervous system and do not express MHC molecules. NK cells primarily target cells that lack MHC molecules, making neurons an incorrect target for NK cells.

D. Bone marrow cells (Option D) are involved in hematopoiesis, the production of blood cells. While bone marrow can be attacked by viruses or malignant cells, the correct target for NK cells is cells that lack MHC molecules, not bone marrow cells as a whole.

**Clinical Pearl:** Understanding the role of natural killer cells in the immune system helps differentiate between innate and adaptive immune responses. NK cells are crucial in recognizing infected or malignant cells, and knowing their targets is essential for clinical decision-making regarding immune system disorders and cancer treatments.

**Correct Answer:** A. Lymphocytes (Option A) are the primary target of natural killer cells due to their role in recognizing and eliminating cells that lack major histocompatibility complex (MHC) molecules. This helps maintain immune system homeostasis and fights infections or cancerous cells. Understanding this concept is essential for understanding immune system functioning and therapeutic interventions in various clinical scenarios.