immune system

This term refers to the biological defense mechanism of an organism. In medical and scientific discourse, it is treated as a singular functional unit, though it comprises various components like white blood cells and lymph nodes. It carries a neutral, technical connotation, though in common parlance, it is often associated with health, resilience, or vulnerability.

Because it describes a specific biological system, it is typically used with the definite article "the" when referring to the general human system or a possessive adjective (e.g., "his immune system") when referring to an individual. It is rarely used in the plural unless discussing the systems of multiple different species or individuals in a comparative study.