Final answer:
The spleen is the organ that comprises white pulp and red pulp, involved in filtering blood and mounting immune responses.
Step-by-step explanation:
The organ made up of white pulp and red pulp is the spleen. The spleen is a vital secondary lymphoid organ known for its role in filtering blood. It contains white pulp, which is rich in lymphocytes and helps to remove antigen-coated bacteria from the blood, and red pulp, which is involved in the storage of blood and removal of damaged red blood cells. The spleen's important functions include the filtration of bloodborne pathogens and the mounting of immune responses, thus earning it the nickname 'filter of the blood.'