What is the primary function of the immune system?

The primary function of the immune system is to fight off infections and diseases. This system is essential for protecting the body from pathogens such as bacteria, viruses, fungi, and parasites. It works through various components, including white blood cells, antibodies, and other immune molecules, to detect and eliminate these harmful agents.

When the immune system identifies a threat, it triggers an immune response, which can involve the activation of various cells that work together to neutralize or destroy the invaders. Memory cells are formed after an infection, allowing the immune system to respond more quickly if the same pathogen is encountered again, highlighting the system’s ability to adapt and learn.

The other options focus on functions that do not pertain to the immune system. For example, hormone production involves the endocrine system, transport of nutrients is handled by the circulatory system, and regulation of temperature and fluid balance is managed primarily by the integumentary and renal systems. These distinctions position the immune system uniquely as the body's defense mechanism against disease and infection.