What is the primary function of the lymphatic and immune system?

The primary function of the lymphatic and immune system is to filter fluid in the body, which is crucial for maintaining fluid balance and supporting the body's immune response. The lymphatic system consists of a network of vessels, nodes, and organs that transport lymph—a fluid containing infection-fighting white blood cells—and is responsible for removing excess fluid from tissues. In addition, it helps filter out pathogens, waste products, and toxins, thereby playing a critical role in defending the body against infections and diseases. This filtering process is essential for the overall functioning of the immune system, which relies on the lymphatic system to effectively monitor and respond to threats to health.

In contrast, the other options, such as regulating hormones, transporting nutrients, and producing red blood cells, pertain to different systems in the body. Hormonal regulation is primarily a function of the endocrine system, nutrient transport is associated with the circulatory system, and red blood cell production occurs in the bone marrow, which is part of the hematopoietic system. Therefore, filtering fluid is specifically tied to the roles of the lymphatic and immune systems, making it the correct choice.