**Core Concept**
The human IgG subclass distribution varies among individuals, with distinct functional and clinical implications. IgG subclass levels can influence the efficacy of vaccines and the susceptibility to certain infections.

**Why the Correct Answer is Right**
IgG1 is the most abundant subclass of IgG, accounting for approximately 60-70% of total IgG in serum. This is due to its ability to bind to Fc receptors on immune cells, facilitating phagocytosis and antibody-dependent cellular cytotoxicity (ADCC). IgG1 also plays a crucial role in the neutralization of pathogens and toxins. The high serum concentration of IgG1 is essential for the maintenance of humoral immunity and the protection against infections.

**Why Each Wrong Option is Incorrect**
**Option A:** IgG2 is the second most abundant subclass, but its serum concentration is significantly lower than IgG1, accounting for approximately 20-30% of total IgG. IgG2 is primarily involved in the response to polysaccharide antigens, such as those found in bacterial capsules.
**Option B:** IgG3 is the least abundant subclass, making up only 5-10% of total IgG. IgG3 has a higher affinity for Fc receptors than IgG1, but its short half-life and limited ability to cross the placenta reduce its overall significance.
**Option D:** IgG4 is the least studied subclass, but it has been implicated in allergic reactions and autoimmune diseases. Its serum concentration is relatively low, and its functional role is still being elucidated.

**Clinical Pearl / High-Yield Fact**
IgG subclass levels can be influenced by genetic factors, age, and environmental exposures. Deficiencies in IgG subclass levels, particularly IgG2, can increase susceptibility to infections and autoimmune diseases.

**Correct Answer: C. IgG1**