## Core Concept
The half-life of an antibody is the time it takes for the concentration of that antibody to decrease by half. Immunoglobulin G (IgG) is the most abundant type of antibody found in blood circulation. Understanding the half-life of IgG is crucial in various clinical and therapeutic contexts.

## Why the Correct Answer is Right
The correct half-life of IgG is approximately **21 days**. This duration allows IgG to provide long-term immunity against infections. IgG's structure, with its two heavy chains and two light chains, enables it to cross the placenta, providing essential immunity to the fetus. The half-life of IgG is influenced by its Fc region, which interacts with the neonatal Fc receptor (FcRn), protecting IgG from degradation.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because the half-life mentioned does not correspond with known immunoglobulin half-lives.
- **Option B:** This option suggests a half-life that is too short for IgG, which does not align with its known long-term immune provision capabilities.
- **Option D:** This option suggests a half-life that is too long for IgG, which does not accurately reflect its turnover rate in the body.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the half-life of IgG is about **21 days**, which is crucial for understanding the duration of action of IgG-based therapies and the timing of immune responses. This half-life allows for the maintenance of long-term immunity and is a factor in the design of immunoglobulin replacement therapies for patients with immunodeficiency disorders.

## Correct Answer: C. 21 days