Natural immunity is defined as which?

• A. Nonspecific cellular and humoral response that operates as the body's first line of defense against pathogens; also called native immunity.
• B. Specific adaptive immunity developed after exposure.
• C. Acquired immunity via vaccines.
• D. Passive immunity from mother to fetus.

Answer: (A)

Natural immunity is the body's nonspecific defenses that are present from birth and do not require prior exposure to a pathogen. It includes physical barriers like skin and mucous membranes, innate immune cells such as neutrophils and macrophages, and mechanisms like inflammation and the complement system. This type acts quickly and broadly against many invaders but doesn’t create memory for a stronger response on future encounters. The other descriptions pertain to adaptive or passive forms of immunity: responses that are specific to particular pathogens after exposure, immunity gained through vaccination, and antibodies transferred from mother to fetus.