Answer:
Primary lymphoid organs: These organs include the bone marrow and the thymus. They create special immune system cells called lymphocytes.
Secondary lymphoid organs: These organs include the lymph nodes, the spleen, the tonsils, and certain tissue in various mucous membrane layers in the body (for instance in the bowel). It is in these organs where the cells of the immune system do their actual job of fighting off germs and foreign substances.
Also...
Bone marrow is a sponge-like tissue found inside the bones. That is where most immune system cells are produced and then also multiply. These cells move to other organs and tissues through the blood. At birth, many bones contain red bone marrow, which actively creates immune system cells. Over the course of our life, more and more red bone marrow turns into fatty tissue. In adulthood, only a few of our bones still contain red bone marrow, including the ribs, breastbone, and pelvis.
