## **Core Concept**
The question tests knowledge of immunoglobulin G (IgG) subtypes and their interaction with the complement system, specifically the alternative pathway. IgG is a crucial antibody isotype that provides long-term immunity against infections. There are four subtypes of IgG: IgG1, IgG2, IgG3, and IgG4, each with distinct biological properties.

## **Why the Correct Answer is Right**
IgG1 and IgG3 are known to activate the classical complement pathway efficiently. However, IgG2 and especially IgG4 are less effective in activating the classical pathway. Among these, **IgG2** can activate the **alternative complement pathway**. This activation is significant for immune defense, especially against encapsulated bacteria. The alternative pathway is one of the three pathways that activate the complement system, which is part of the innate immune system.

## **Why Each Wrong Option is Incorrect**
- **Option A (IgG1):** Primarily activates the classical complement pathway, not the alternative pathway.
- **Option C (IgG3):** Also activates the classical complement pathway efficiently but is not the best answer for alternative pathway activation.
- **Option D (IgG4):** Has a low ability to activate the complement system through either the classical or alternative pathways.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that IgG2 deficiency is associated with an increased susceptibility to infections, particularly with encapsulated bacteria, due to its role in activating the alternative complement pathway and its involvement in antibody responses against polysaccharide antigens.

## **Correct Answer: B. IgG2.**