Which of the following is not a malfunction of the immune system?

• A. Allergy
• B. Immunodeficiency
• C. Autoimmune disease
• D. Immunity by vaccination

Answer: (D)

The immune system can function improperly in ways that cause disease, but vaccination is not one of those malfunctions. Vaccination intentionally exposes the immune system to safe, prepared pieces of a pathogen or inactivated pathogen to train it. This prompts the body to produce specific antibodies and memory cells so it can respond quickly if real exposure occurs later, providing protection rather than dysfunction.

Allergy reflects an overreactive immune response to harmless substances, demonstrating hypersensitivity. Immunodeficiency means the immune system isn’t able to mount a strong enough response against real threats. Autoimmune disease occurs when the immune system mistakenly attacks the body's own tissues. In contrast, vaccination strengthens and directs the immune response in a controlled way, so it’s not a malfunction.