What role does the immune system play in human health?

The immune system is a critical component of human health, primarily responsible for protecting the body against diseases. This includes defending against pathogens such as bacteria, viruses, fungi, and parasites. When the immune system encounters these harmful agents, it activates a complex response involving white blood cells and various proteins, which work together to identify and eliminate threats. This protective function helps to prevent infections and diseases, maintaining overall health.

Other options, while important for overall bodily function, do not pertain directly to the primary role of the immune system. Digestion of food is the responsibility of the digestive system, regulation of body temperature is managed by the thermoregulatory processes within the body, and the distribution of nutrients primarily falls under the circulatory system's purview. Thus, the immune system's main function as a defender against diseases is essential for sustaining health and well-being.