**Question:** Difference between natural killer (NK) and cytotoxic cells is -

A. NK cells are responsible for cell lysis, while cytotoxic T cells are involved in antibody production.
B. NK cells target cells infected with viruses, while cytotoxic T cells focus on cancerous and infected cells.
C. NK cells are a part of innate immunity, while cytotoxic T cells are part of adaptive immunity.
D. NK cells are cytotoxic cells, while cytotoxic T cells are responsible for releasing cytokines.

**Correct Answer:**

C. NK cells are a part of innate immunity, while cytotoxic T cells are part of adaptive immunity.

**Core Concept:**

Natural killer (NK) cells and cytotoxic T cells are both cytotoxic lymphocytes, which are crucial components of the immune system. However, they differ in their functions and roles in the body.

**Why the Correct Answer is Right:**

Natural killer (NK) cells are a type of lymphocyte found in the innate immune system. They are equipped with cytotoxic granules that allow them to directly lyse (destroy) virus-infected cells and tumor cells without prior sensitization. In contrast, cytotoxic T cells are a part of the adaptive immune system and require antigen recognition and prior sensitization before they become activated. Once activated, cytotoxic T cells are specialized in lysing virus-infected and cancerous cells and producing cytokines, which modulate the immune response.

**Why Each Wrong Option is Incorrect:**

A. This statement is partially correct, as NK cells are involved in cell lysis, but cytotoxic T cells are not primarily focused on antibody production. Rather, cytotoxic T cells are responsible for producing cytokines, which regulate the immune response.

B. While NK cells target virus-infected cells, cytotoxic T cells are mainly involved in killing cancerous cells and virus-infected cells after antigen recognition and prior sensitization.

D. This statement is incorrect, as NK cells are cytotoxic cells, but cytotoxic T cells are involved in cytokine production, not cytokine release.

**Clinical Pearls:**

Understanding the difference between NK cells and cytotoxic T cells is essential in understanding immune system function. NK cells act as a first line of defense in eliminating virus-infected and tumor cells, while cytotoxic T cells are involved in regulating the immune response via cytokines following antigen recognition and sensitization. This distinction is crucial in clinical practice, as misinterpreting the roles of these lymphocytes may lead to misdiagnosis and inappropriate treatment decisions.