Human IgG subclasses: Into how many distinct IgG classes (subclasses) is human IgG divided?

Introduction / Context:
Human IgG antibodies are subdivided into subclasses with different hinge regions, serum abundance, and effector functions. Recognizing the number and names aids in interpreting laboratory results and understanding vaccine responses.

Given Data / Assumptions:

• Human IgG subclasses: IgG1, IgG2, IgG3, IgG4.
• Each subclass has unique properties (e.g., complement activation strength).

Concept / Approach:
Subclassification is based on differences in heavy chain gamma constant regions (γ1–γ4). These differences affect Fc-mediated functions, placental transfer efficiency, and half-life.

Step-by-Step Solution:
List subclasses: 1, 2, 3, 4. Confirm no fifth subclass exists in humans. Select the option indicating four. Note: nomenclature sometimes written as IgG1–IgG4.

Verification / Alternative check:
Immunology references and clinical assay reports (e.g., IgG subclass panels) enumerate four distinct subclasses.

Why Other Options Are Wrong:
Two or three undercount; five overcounts; one ignores known subclass diversity.

Common Pitfalls:
Confusing human subclass count with other species or with Ig heavy-chain classes (IgG, IgA, IgM, IgD, IgE).

Final Answer:
Four IgG classes.