What tissue acts as a primary site of immune responses by filtering antigens from blood?

The spleen plays a crucial role in immune responses by filtering antigens from the blood. It is an essential organ of the lymphatic system that helps identify and respond to pathogens. The spleen contains specialized areas known as white pulp, which is rich in lymphocytes, including T and B cells. When blood flows through the spleen, these immune cells can encounter antigens present in the blood, leading to the activation of an immune response.

Additionally, the spleen is involved in the removal of old or damaged red blood cells and recycling iron, but its primary role in this context is the filtering of blood to capture pathogens and facilitate an efficient immune response. This function distinguishes it from other options, such as the bone marrow, which is primarily responsible for the production of blood cells, and the thymus, which is mainly involved in T cell development and maturation. The liver also has immune functions, particularly in filtering and processing blood, but it does not specialize in the direct immune filtering of antigens in the same way the spleen does.