Which organ system includes the spleen and thymus?

The correct answer is the immune system because both the spleen and thymus are integral components of this system, which is responsible for defending the body against pathogens. The spleen plays a crucial role in filtering the blood, removing old or damaged red blood cells, and producing lymphocytes, which are vital for immune responses. The thymus is essential for the development of T-lymphocytes (T cells), which are critical for adaptive immunity and help the body recognize and attack specific pathogens.

In contrast, the nervous system is primarily involved with signal transmission throughout the body to coordinate movement and physiological responses, the digestive system focuses on the breakdown and absorption of nutrients, and the endocrine system regulates bodily functions through hormones. While the spleen and thymus play roles in overall health and bodily function, their specific contributions to immune defense are what categorically place them within the immune system.