**Core Concept**
The primary immunoglobulin (Ig) response following an infection involves the activation of B cells, which differentiate into plasma cells producing specific antibodies. The initial Ig to appear is crucial for providing immediate protection against pathogens.

**Why the Correct Answer is Right**
The correct answer is IgM, as it is the first immunoglobulin to be produced in response to an infection. This is due to the fact that IgM is produced by B cells that have not yet undergone class switching, and it is the most efficient antibody in activating the complement system. IgM's pentameric structure allows it to bind to multiple epitopes, enhancing its ability to neutralize pathogens. The production of IgM is a hallmark of the early immune response, providing immediate protection against infections.

**Why Each Wrong Option is Incorrect**
**Option A:** IgG is the most abundant immunoglobulin in the serum and provides long-term immunity, but it is not the first to appear following an infection.
**Option B:** IgE is primarily involved in allergic reactions and parasitic infections, and it is not the initial immunoglobulin produced in response to a typical infection.
**Option C:** IgA is the main immunoglobulin found in mucosal secretions and provides protection against infections in mucosal surfaces, but it is not the first to appear following an infection.

**Clinical Pearl / High-Yield Fact**
Remember that the production of IgM is a crucial indicator of an active immune response, and its presence can be used to diagnose infections such as rheumatic fever or infectious mononucleosis.

**Correct Answer:** .