The serum concentration of which of the following human IgG subclass is maximum ?
**Core Concept**
The human immune system produces five classes of immunoglobulins (Ig): IgG, IgM, IgA, IgE, and IgD. IgG is the most abundant class in the serum and is further divided into four subclasses: IgG1, IgG2, IgG3, and IgG4. Each subclass has distinct biological properties and plays a different role in immune defense.
**Why the Correct Answer is Right**
IgG1 is the most abundant subclass of IgG in human serum, accounting for approximately 60-70% of the total IgG. This is due to its high production rate by activated B cells and its long half-life in the circulation. IgG1 is particularly effective in neutralizing pathogens and toxins, making it a crucial component of the humoral immune response. The high concentration of IgG1 in the serum is also important for its role in providing passive immunity to newborns through maternal transfer.
**Why Each Wrong Option is Incorrect**
**Option A:** IgG2 is the second-most abundant subclass of IgG, but its concentration is significantly lower than IgG1, accounting for approximately 20-30% of the total IgG.
**Option B:** IgG3 is the least abundant subclass of IgG, accounting for only about 5-10% of the total IgG, and is primarily involved in the immune response to viral infections.
**Option C:** IgG4 is the least abundant subclass of IgG, accounting for about 3-5% of the total IgG, and is often associated with allergic reactions and parasitic infections.
**Clinical Pearl / High-Yield Fact**
It's essential to note that the ratio of IgG subclasses can be altered in certain disease states, such as chronic infections or autoimmune disorders, which can impact the effectiveness of the immune response.
**Correct Answer:** C.