The most important role of the immune system is to keep foreign substances from entering the blood circulation.

• A. True
• B. False
• C. It varies
• D. Not sure

Answer: (B)

Protection against pathogens relies on both physical barriers and immune responses. While skin and mucous membranes act as the first line to keep many foreign substances from entering the body, the immune system’s central job is to detect non-self or abnormal cells and mount a coordinated response to eliminate them, especially when they breach those barriers. This involves rapid innate defenses such as phagocytes and inflammation, and later, specific adaptive defenses like antibodies and T cells that target particular invaders. Since keeping substances out of the blood entirely is not how the immune system operates—barriers do that screening, but infections are fought after entry—the statement is not accurate.