In the Lymphatic System, Most Invading Pathogens are First Encountered

Question

In the lymphatic system, most invading pathogens are first encountered by ________.
A) primary lymphatic structures
B) tertiary lymphatic structures
C) the skin
D) secondary lymphatic structures
E) the liver

Accepted Answer

Secondary lymphatic structures, such as lymph nodes, spleen, and tonsils, are where most invading pathogens are first encountered by immune cells, which then initiate an immune response. Primary lymphatic structures, such as the bone marrow and thymus, produce and mature immune cells, but do not directly encounter pathogens. Tertiary lymphatic structures and the liver are not as directly involved in the initial encounter with pathogens. The skin may help to prevent pathogens from entering the body, but it does not initiate an immune response like the lymphatic system.