## Core Concept
An immunoglobulin, also known as an antibody, is a Y-shaped protein molecule produced by B cells (plasma cells) that plays a crucial role in the immune system by recognizing and binding to specific antigens. Each immunoglobulin molecule is composed of multiple polypeptide chains.

## Why the Correct Answer is Right
A single immunoglobulin molecule contains **two heavy chains** and **two light chains**, which are linked together by disulfide bonds to form a Y-shaped structure. This structure allows the antibody to bind to specific antigens through its variable regions, which are located at the tips of the Y-shaped molecule. The correct answer reflects this basic structural characteristic of immunoglobulins.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because it does not accurately represent the composition of a single immunoglobulin molecule.
* **Option B:** This option is incorrect for the same reason as Option A; it does not correctly specify the number of heavy and light chains in an immunoglobulin molecule.
* **Option D:** This option is incorrect because, like Options A and B, it does not accurately describe the structural composition of a single immunoglobulin molecule.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the Y-shaped structure of immunoglobulins, composed of two heavy chains and two light chains, allows for the **divalent binding** to antigens, which is crucial for the neutralization and removal of pathogens from the body. This structural feature is fundamental to the function of antibodies in the immune response.

## Correct Answer: C. Two heavy chains and two light chains.