**Question:** All of the following are a part of the innate immunity except -

A. Complement system
B. Natural killer cells
C. Antibodies
D. Adaptive immunity

**Core Concept:** Innate immunity is the body's first line of defense against pathogens, providing a rapid, nonspecific response. It comprises various components, including complement system, natural killer cells, and antibodies. Adaptive immunity, on the other hand, is a slower, specific response involving antibodies and memory cells.

**Why the Correct Answer is Right:** D. Adaptive immunity is a part of acquired immunity and not innate immunity. It takes time for the immune system to produce specific antibodies and activate memory cells in response to previously encountered pathogens.

**Why Each Wrong Option is Incorrect:**

A. Complement system: Complement system is a crucial component of innate immunity, contributing to pathogen recognition, opsonization, and direct lysis of pathogens.

B. Natural killer cells: Natural killer cells are also part of the innate immune response, particularly in recognizing and eliminating virus-infected cells and tumor cells.

C. Antibodies: Antibodies, also known as immunoglobulins, are proteins produced by B cells in response to specific antigens encountered previously. They are a key component of adaptive immunity, not innate immunity.

**Clinical Pearl / High-Yield Fact:**

The complement system plays a vital role in both innate and adaptive immunity, enhancing the immune response through opsonization, direct pathogen lysis, and chemotaxis. However, the core components of innate immunity - natural killer cells and antibodies - are specific to adaptive immunity.

In contrast, innate immunity includes broader components like cytokines, chemokines, and phagocytes that provide rapid and nonspecific defense against pathogens without prior exposure.