**Core Concept**
IgG is the most abundant class of immunoglobulins in human serum, providing long-term defense against infections. It has four subclasses: IgG1, IgG2, IgG3, and IgG4, each with distinct biological properties.

**Why the Correct Answer is Right**
IgG1 is the most abundant subclass of IgG, accounting for approximately 60-70% of total IgG in human serum. This is due to its high affinity for the Fc receptor on immune effector cells, which enhances its ability to activate the complement system and recruit immune cells to sites of infection. IgG1 also has a longer half-life compared to other subclasses, which contributes to its high serum concentration.

**Why Each Wrong Option is Incorrect**
**Option A:** IgG2 is the second most abundant subclass of IgG, but it has a lower affinity for the Fc receptor and a shorter half-life compared to IgG1.
**Option B:** IgG3 has a higher affinity for the Fc receptor compared to IgG2, but it is less abundant in serum due to its shorter half-life and more rapid clearance.
**Option C:** IgG4 has the lowest affinity for the Fc receptor and is the least abundant subclass of IgG, making it unlikely to have the maximum serum concentration.

**Clinical Pearl / High-Yield Fact**
IgG levels can be used as a marker of immune status, with decreased levels indicating immunodeficiency or chronic disease. Additionally, the ratio of IgG subclasses can provide insight into the type of immune response, with an increased IgG1/IgG2 ratio suggesting a Th1-mediated response.

**Correct Answer: C. IgG3.**