Immune system organs, tonsils, ganglia, spleen, red bone marrow. The immune system consists of organs connected by blood and lymph vessels. These organs are meeting places between lymphocytes and microbes circulating in the blood and lymph. Microbes are pathogens such as bacteria and viruses. These immune cell storage organs are the tonsils, lymph nodes and spleen shown in green. The thymus and the bone red marrow represented in orange is the organ of formation of immune cells. Thymus plays a very important role in the establishment of the immune system in children by ensuring the maturation of T lymphocytes (thymocytes)