The largest of these are the macrophages , which appear almost like amebas in their ability to move and to consume and digest bacteria or other foreign cells. These are large cells that are involved in allergic reactions, producing histamine among other chemicals used to fight infection. There are several types of granulocytes, so called because they resemble grains under a microscope, including the eosinophils and basophils, which like the mast cells are part of the allergic reaction. Another type of granulocyte, the neutrophil , is a bacteria fighter. One place where these cells are stored until needed is the spleen .
Blood passes through the spleen, which has a concentration of lymphocytes to attack intruders.
Immune system cells also are found throughout the lymphatic system, especially in the nodes, and throughout the bloodstream. Much of the work of the immune system is carried out by the lymphatic system, thymus , and spleen , but bone marrow is the source of the white blood cells. The skin is not only the first line of defense, it also helps to train some white blood cells. The immune system guards against infection in part by its amazing ability to remember what it has encountered in the past. The B cells are famous for the learned response known as antibodies.