Despite these frontline defenses, many germs do succeed in penetrating the tissues and entering the bloodstream. Protein complexes in the blood such as antibodies and interferon do their best to destroy them. In many tissues, such as the liver, lungs, brain, spleen, lymph nodes, and bone marrow, fixed macrophages lie in wait for them. These are cells capable of gobbling up bacteria and dead matter, an activity known as phagocytosis. The spleen, the largest mass of lymphatic tissue, has a major phagocytic function, cleansing the blood of foreign organisms, worn-out red blood cells, and platelets.
Other white cells made in the bone marrow and lymphoid tissue move through the blood to engulf bacteria. First to arrive at the site of an impending infection are a type of granular white cell called neutrophils. Granular refers to their grainy structure inside. These are followed by agranular white cells called monocytes, which enlarge and become macrophages on the way. They are called wandering macrophages because they leave the blood and enter the tissues to do battle with adversaries. These phagocytic cells are undiscriminating about what they swallow. They are part of the general, non-specific system of resistance to disease.
