What is the primary role of the spleen in the immune system?

The spleen plays a crucial role in the immune system primarily through its function in the filtration of blood and initiation of immune responses. It is responsible for filtering out old or damaged red blood cells from the bloodstream, as well as pathogens and debris. This filtration process helps maintain healthy blood composition and ensures that pathogens are detected and dealt with.

Additionally, the spleen is a site where immune responses can be triggered. It contains various types of immune cells, including lymphocytes and macrophages, which are essential for identifying and responding to infections. When antigens from pathogens enter the blood, they can be recognized by these immune cells in the spleen, leading to an immune response designed to eliminate the threat.

This role of the spleen is fundamental, given its position in the circulatory system and its ability to interact with blood-borne pathogens, making option B the correct choice. Other functions mentioned in the other options do not pertain to the primary immunological roles of the spleen. For instance, insulin production is associated with the pancreas, blood pressure regulation is primarily managed by the cardiovascular system, and calcium storage is not a primary function of the spleen but rather of bones and other tissues.