**Core Concept**
Human IgG antibodies are the most abundant type of antibodies in the blood, and they are further divided into four subclasses: IgG1, IgG2, IgG3, and IgG4. Each subclass has distinct properties and functions, and their serum concentrations vary.

**Why the Correct Answer is Right**
IgG1 is the most abundant subclass of IgG antibodies in human serum, accounting for approximately 60-70% of the total IgG concentration. This is due to its ability to bind to the Fc receptor on immune cells, such as macrophages and neutrophils, which enhances its effector functions, including phagocytosis and antibody-dependent cellular cytotoxicity (ADCC). The high serum concentration of IgG1 is also reflected in its ability to cross the placenta, providing passive immunity to the fetus.

**Why Each Wrong Option is Incorrect**
**Option B:** IgG2 is the second most abundant subclass of IgG antibodies, but its serum concentration is significantly lower than that of IgG1, accounting for approximately 20-30% of the total IgG concentration.

**Option C:** IgG3 is the least abundant subclass of IgG antibodies, but it has the highest affinity for the Fc receptor, which enhances its effector functions. However, its serum concentration is much lower than that of IgG1.

**Option D:** IgG4 is the least abundant subclass of IgG antibodies, and its serum concentration is lowest among all the IgG subclasses. It is also known for its ability to form immune complexes, which can contribute to allergic reactions and anaphylaxis.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the serum concentration of IgG antibodies can be affected by various factors, including age, sex, and disease states, such as infections and autoimmune disorders.

**✓ Correct Answer: A. IgG1**