Which system is actively involved in the immune response to pathogens?

The immune system is specifically responsible for detecting and responding to pathogens, such as bacteria, viruses, and fungi. It consists of a complex network of cells, tissues, and organs that work together to identify and eliminate these harmful invaders. This system includes white blood cells, antibodies, and various signaling molecules that coordinate the body's defense mechanisms.

When a pathogen enters the body, the immune system activates specific responses to target and destroy the invader. For example, macrophages may engulf and digest pathogens, while lymphocytes, such as T cells and B cells, have specialized roles in recognizing and responding to specific antigens presented by the pathogens. This targeted response is crucial for preventing infections and maintaining overall health.

In contrast, the other systems mentioned—the respiratory, endocrine, and muscular systems—do not play a direct role in the immune response. The respiratory system primarily focuses on gas exchange and maintaining oxygen levels in the body, the endocrine system regulates hormones and bodily functions, while the muscular system facilitates movement and physical activity. These systems do not have the specialized functions necessary for immune surveillance and pathogen elimination, which is why the immune system is the correct answer.