**Core Concept**
The sedimentation coefficient classifies immunoglobulin (Ig) molecules based on their size and shape in an ultracentrifuge. IgE is a monomeric immunoglobulin with a specific sedimentation coefficient that reflects its molecular weight and density.

**Why the Correct Answer is Right**
IgE has a sedimentation coefficient of **8S**, which is determined by its molecular size and structure. Unlike IgG (11S) or IgA (7S), IgE is a monomeric antibody with a smaller molecular weight and a unique structure that results in its 8S classification. This value is consistent across standard immunoglobulin sedimentation coefficient tables and is critical in diagnosing immunoglobulin deficiencies or hypersensitivity disorders.

**Why Each Wrong Option is Incorrect**
Option A: IgE is not 7S — that is the sedimentation coefficient of IgA, not IgE.
Option C: 11S is the sedimentation coefficient of IgG, not IgE.
Option D: 20S refers to larger proteins like IgM (which is 19S), not IgE.

**Clinical Pearl / High-Yield Fact**
IgE is the only immunoglobulin with a 8S sedimentation coefficient; remember: **IgA = 7S, IgG = 11S, IgM = 19S, IgE = 8S**. This helps differentiate serum immunoglobulins in laboratory settings and is key in diagnosing allergies and parasitic infections.

✓ Correct Answer: B. 8S