What is the primary function of the immune system?

The primary function of the immune system is to defend against pathogens and disease. This system is a complex network of cells, tissues, and organs that work together to identify and neutralize harmful invaders, such as bacteria, viruses, and fungi. It distinguishes between the body’s own cells and foreign substances, mounting an appropriate response to eliminate threats while maintaining the health of the organism.

For example, when a pathogen breaches the body's barriers, specialized immune cells, such as macrophages and lymphocytes, are activated. These cells can recognize the pathogen, destroy it, and remember it for future encounters, which contributes to the body’s immunity. This protective role is crucial for preventing infections and fighting off diseases, ensuring the overall well-being of the organism.

The other options describe functions unrelated to the immune response. The transport of oxygen is primarily the role of the circulatory system, regulating body temperature is managed by the body's thermoregulatory mechanisms, and aiding in digestion involves the gastrointestinal system. Hence, these options do not align with the primary role of the immune system, which is specifically focused on defense against disease and infection.