**Core Concept**
Glutathione is a tripeptide antioxidant molecule composed of glutamine, cysteine, and glycine. It plays a crucial role in cellular defense mechanisms, particularly in protecting cells from oxidative stress and damage caused by reactive oxygen species (ROS).

**Why the Correct Answer is Right**
Glutathione functions as a reducing agent, donating electrons to oxidized molecules and thereby neutralizing ROS. This process is facilitated by the sulfhydryl group (-SH) of cysteine, which is essential for glutathione's antioxidant activity. The enzyme glutathione S-transferase also plays a role in conjugating glutathione to electrophilic compounds, further enhancing its detoxifying properties.

**Why Each Wrong Option is Incorrect**
**Option A:** Glutathione is not a protein, as it is a small peptide composed of only three amino acids.
**Option B:** While glutathione does have some role in immune function, it is not primarily an antibody.
**Option C:** Glutathione is not a neurotransmitter, as its primary function is not related to neuronal signaling.

**Clinical Pearl / High-Yield Fact**
Glutathione levels are often depleted in conditions such as liver disease, cancer, and HIV infection, highlighting its importance in maintaining cellular homeostasis.

**Correct Answer:** D. A tripeptide antioxidant.