**Core Concept**
Protamine, an antidote for heparin, is a positively charged molecule that binds to the negatively charged heparin molecule, effectively neutralizing its anticoagulant activity. This interaction is based on the chemical properties of protamine and heparin.

**Why the Correct Answer is Right**
Protamine exerts its effect by forming a stable complex with heparin through a chemical reaction. This reaction occurs due to the strong electrostatic attraction between the positively charged amino groups of protamine and the negatively charged sulfate groups of heparin. The binding of protamine to heparin results in the formation of a stable salt-like complex, which effectively neutralizes the anticoagulant activity of heparin. This mechanism is an example of chemical antagonism, where the antidote (protamine) reacts chemically with the drug (heparin) to neutralize its effect.

**Why Each Wrong Option is Incorrect**
**Option A:** Competitive antagonism is incorrect because protamine does not compete with heparin for the same receptor site; instead, it binds to heparin itself.
**Option C:** Non-competitive antagonism is incorrect because protamine does not alter the receptor's affinity for heparin or its ability to activate the coagulation pathway; it simply neutralizes heparin through chemical binding.
**Option D:** Toxic reaction is incorrect because the action of protamine is not based on inducing a toxic effect on the body; rather, it is a specific chemical reaction that neutralizes heparin.

**Clinical Pearl / High-Yield Fact**
It is essential to note that protamine is a highly basic compound and can cause a rapid increase in blood pressure if administered too quickly. Therefore, it should be administered slowly and under close monitoring to prevent complications.

**✓ Correct Answer: B. Chemical antagonism**