Natural immunity is best described as which of the following?

• A. Nondeliberate exposure to antigens
• B. Immunity transferred from one individual to another
• C. Deliberate exposure to antigens
• D. Immune system responds to a harmful agent

Answer: (A)

Natural immunity, or innate immunity, refers to the body's built-in, nonspecific defenses that are always present and do not require deliberate exposure to a specific antigen. It includes barriers like the skin and mucous membranes, phagocytic cells, natural killer cells, the inflammatory response, and the complement system. Because these defenses are active from birth and don’t depend on a prior encounter with a particular antigen, the description of nondeliberate exposure fits best. In contrast, immunity transferred from one individual to another is passive immunity (such as maternal antibodies), deliberate exposure to antigens describes active, adaptive immunity (like vaccination or infection), and the immune system simply “responds to a harmful agent” is a general statement about immune action that doesn’t specify the innate, nonspecific nature of natural immunity.