Light microscopy of the spleen, showing the internal structure using the reticulin stain which uses silver impregnation to highlight collagen fibres. Blood vessel walls stain darkly. Many circular lymphoid follicles, making up the white pulp of the spleen, are revealed due to their supporting frameworks of collagen fibres and a neutral red counterstain to stain the cells, chiefly lymphocytes and other immune cells. The spleen is a lymphoid organ that filters blood to detect and destroy pathogens, and to remove worn out red blood cells. The reticular fibres form a dense meshwork between the lymphoid follicles, and constitute the red pulp of the spleen. Magnification x7 when printed at 10 cm.