The macrophages arrive afterwards, to collect the evidence and destroy any microbes that may have escaped the microphage guards. Both microphages and macrophages come from the bone marrow. But the bone marrow also makes stem cells, which move to different parts of the body. These include other important immune organs like the thymus gland, spleen, lymph nodes, tonsils, appendix and Peyer's patches in the intestinal wall. Once settled into the organ, the stem cells produce white blood cells called lymphocytes.
Cells produce more cells by dividing into two cells.
The daughter cells divide again, and so the process continues. The stem cells that go into the thymus gland divide very quickly, to produce a large number of daughter cells, most of which don't survive. Those that do, then leave the thymus gland and travel between the other immune organs at will. But they never return to the thymus. And they learn special skills in the thymus gland which are very useful in fighting off foreign microbes.