**Core Concept**
A hapten is a small molecule that, while it cannot induce an immune response by itself, can bind to a larger carrier protein to form an immunogenic complex. This complex can then stimulate a specific immune response, leading to the production of antibodies against the hapten.

**Why the Correct Answer is Right**
Haptens work by binding to receptors on immune cells, such as B cells and T cells, through a process known as cross-linking. This binding event activates the immune cells, leading to the activation of other immune cells, such as T helper cells, which then stimulate the production of antibodies against the hapten. The key to hapten action is that the hapten itself is not immunogenic, but it becomes immunogenic when it binds to a carrier protein, such as a protein on a red blood cell.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the nature of haptens. While some haptens may be small enough to be metabolized by the body, this is not a defining characteristic of haptens.
* **Option B:** This option is incorrect because it implies that haptens are always toxic, which is not the case. Haptens can be found in a wide range of substances, including food additives, medications, and environmental toxins.
* **Option C:** This option is incorrect because it implies that haptens are always antigens, which is not the case. Antigens are substances that can induce an immune response on their own, whereas haptens require a carrier protein to become immunogenic.

**Clinical Pearl / High-Yield Fact**
The concept of haptens is important in immunology because it helps to explain how some substances can cause allergic reactions, even if they are not inherently immunogenic. For example, penicillin is a hapten that can cause an allergic reaction when it binds to a carrier protein on a red blood cell.

**Correct Answer:** C.