**Core Concept**
Human immunoglobulins, also known as antibodies, are crucial proteins in the adaptive immune system that recognize and bind to specific antigens. They are divided into five main classes or isotypes based on their structure and function.

**Why the Correct Answer is Right**
The correct classification of human immunoglobulins is based on the structure of their heavy chains. The five classes are IgA, IgD, IgE, IgG, and IgM. IgG is the most abundant class in the blood and provides long-term immunity against infections. IgM is the first antibody produced in response to an infection and is primarily involved in the primary immune response. IgA is the main antibody found in mucosal surfaces, such as the respiratory and gastrointestinal tracts, and provides protection against infections in these areas. IgE is primarily involved in allergic reactions and parasitic infections. IgD is found on the surface of mature B cells and plays a role in the activation of these cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a valid classification system for human immunoglobulins.
**Option B:** This option is incorrect because it does not correctly identify the main classes of human immunoglobulins.
**Option C:** This option is incorrect because it does not accurately describe the classification system for human immunoglobulins.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the five classes of human immunoglobulins have distinct functions and are found in different locations within the body. IgG is the only class that can cross the placenta, providing immunity to the fetus.

**Correct Answer: D. The five main classes are IgA, IgD, IgE, IgG, and IgM.**