Light microscopy of primate spleen showing white pulp (blue) and red pulp (pink). The spleen is part of the body's immune system and classified as a lymphoid organ. It filters blood and serves to screen the blood for the presence of foreign antigens that may be a threat to the body's health. The red pulp is formed like a sieve through which blood vessels pass. The white pulp is formed of lymphoid cells, mainly lymphocytes, which initiate an immune response if harmful agents in the blood are detected. The spleen also removes and destroys worn out red blood cells. Magnification x9 when printed at 10 cm.