What are the two major categories of immune mechanisms?

• A. Innate (nonspecific) immunity and adaptive (specific) immunity.
• B. Humoral and cellular immunity.
• C. Active and passive immunity.
• D. Naturally acquired and artificially acquired immunity.

Answer: (A)

The immune response operates in two broad groups: innate immunity, which provides immediate, nonspecific defenses such as barrier tissues, phagocytes, inflammation, and the complement system; and adaptive immunity, which develops a targeted, antigen-specific response through B and T lymphocytes and includes immunological memory for faster protection on re-exposure. The other terms describe either subdivisions within adaptive immunity (humoral vs cellular) or how immunity is acquired (active vs passive; natural vs artificial), rather than the two overarching modes of defense.