Which phrase best describes "immune response"?

• A. The body's reaction against invading agents
• B. The body's reaction to extremes of temperature or humidity
• C. A resistance to disease
• D. A type of vaccination

Answer: (A)

The immune response is the body's defense against invading agents. It starts when the immune system recognizes foreign substances such as bacteria, viruses, or parasites and then activates a coordinated attack that may involve inflammation, phagocytosis by immune cells, production of antibodies, and activation of cytotoxic cells. This describes the actual process of fighting off invaders, not just a general condition or a preventative measure.

The other ideas describe related concepts but not the active mechanism: reacting to extreme temperatures or humidity is a stress or thermoregulatory response, not an immune process; being resistant to disease is an outcome or state rather than the dynamic response itself; and a vaccination is a way to stimulate immunity in advance, not the body's immediate response to an invasion.