**Core Concept**
The question tests the understanding of **immunoglobulins**, also known as antibodies, which are crucial for the immune system. Immunoglobulins are proteins produced by B cells that help fight infections. There are five classes of immunoglobulins: IgA, IgD, IgE, IgG, and IgM.

**Why the Correct Answer is Right**
IgE is the least abundant, making up only a small fraction of the immunoglobulins in human serum. It plays a key role in **allergic reactions** and **parasite infections**. Despite its low concentration, IgE is significant due to its involvement in **type I hypersensitivity reactions**.

**Why Each Wrong Option is Incorrect**
**Option A:** IgG is the most abundant immunoglobulin in human serum, making it incorrect.
**Option B:** IgM is also relatively abundant, especially in the early stages of infection.
**Option C:** IgA is important for mucosal immunity and is not the least abundant.

**Clinical Pearl / High-Yield Fact**
Remember, IgE is associated with **allergic reactions**, such as anaphylaxis, and its levels can be elevated in individuals with **atopic diseases** like asthma.

**Correct Answer:** D. IgE.