Organs of the immune system make cells, that either contribute in the immune response, or act as sites for the immune function. Major Organs of the Immune System. The main organs in the immune system are the bone marrow, thymus, spleen and lymph nodes.Let us have a look at these immune system organs and functions. Bone Marrow: All the cells of the human immune system are formed in the bone marrow, found within the bones, by a process called hematopoiesis.-Cells: The main function of T-cells or T lymphocytes is intensifying immune system responses. They do this by secreting specialized factors, which in turn activate other white blood cells, to fight off infections. T-cells are again subdivided into different types.