The systems that both directly work to develop resistance to pathogens are the ______ systems.

• A. Lymphatic and Immune
• B. Digestive and Endocrine
• C. Respiratory and Nervous
• D. Circulatory and Integumentary

Answer: (A)

Resistance to pathogens is driven by the immune system, with the lymphatic system playing a crucial supporting role. The immune system consists of cells and tissues that identify and neutralize invaders—like T cells, B cells, antibodies, macrophages—and carry out targeted attacks against bacteria, viruses, and other pathogens. The lymphatic system, through its network of vessels, lymph nodes, the spleen, and thymus, moves immune cells to sites of infection, filters pathogens from the body, and helps present antigens to activate immune responses. Together, they directly develop resistance. Other systems contribute to defense in other ways—for example, barrier and barrier-related functions of the skin, or transport and coordination roles of the circulatory, respiratory, digestive, endocrine, and nervous systems—but they do not constitute the primary pair that directly builds immune resistance like the lymphatic and immune systems do.