**Core Concept**
Immunoglobulins, also known as antibodies, are glycoprotein molecules produced by B lymphocytes (B cells) in response to the presence of specific antigens. They play a crucial role in the adaptive immune response by recognizing and binding to specific antigens, thereby neutralizing or removing them from the body.

**Why the Correct Answer is Right**
The correct answer is related to the structure and function of immunoglobulins. Immunoglobulins have a Y-shaped structure, consisting of two heavy chains and two light chains, which provide the binding sites for antigens. The Fab region (fragment, antigen-binding) of the immunoglobulin contains the antigen-binding sites, while the Fc region (fragment, crystallizable) is responsible for interacting with immune cells such as macrophages and neutrophils. This interaction facilitates the removal of antigens and the activation of immune responses.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe a characteristic of immunoglobulins. Immunoglobulins are not primarily involved in cell-to-cell signaling.

**Option B:** This option is incorrect because it does not accurately describe the function of immunoglobulins. While immunoglobulins can activate complement, this is not their primary function.

**Option C:** This option is incorrect because it does not accurately describe the structure of immunoglobulins. Immunoglobulins do not have a single polypeptide chain.

**Clinical Pearl / High-Yield Fact**
Immunoglobulins are classified into five classes: IgA, IgD, IgE, IgG, and IgM. IgG is the most abundant class of immunoglobulins and provides long-term immunity against infections.

**Correct Answer:** D