Final answer:
The spleen is the lymphoid organ responsible for filtering blood, possessing regions known as white and red pulp for triggering immune responses and removing aged blood cells, respectively.
Step-by-step explanation:
The lymphoid organ responsible for filtering blood is the spleen.
The spleen plays a critical role as a secondary lymphoid organ, where it performs several functions related to the immune system and blood filtration.
It contains white pulp and red pulp; the white pulp is where pathogens are likely to encounter lymphocytes, triggering an adaptive immune response, while the red pulp is responsible for removing aged or damaged blood cells.
As such, the spleen not only filters lymph but also directly filters blood, significantly contributing to the body's defense mechanisms against infections.