What is the primary function of the immune system?

The primary function of the immune system is to defend the body against pathogens and disease. This intricate network of cells, tissues, and organs works together to identify and neutralize harmful invaders such as bacteria, viruses, fungi, and parasites. The immune system employs various strategies, including the production of antibodies, activation of immune cells, and the expression of various proteins that signal the immune response, all of which contribute to maintaining health and preventing infections.

In contrast to the other mentioned functions, transporting nutrients and oxygen to cells relates primarily to the circulatory system, not the immune system. The role of hormone production belongs to the endocrine system, which regulates various bodily functions but does not primarily focus on pathogen defense. Similarly, while regulating body temperature is a vital function, it is managed by the thermoregulatory mechanisms within the nervous and endocrine systems rather than the immune system. Hence, the unique and specialized role of disease defense solidifies the answer as correct.