Lymphatic system in children, lymphatic network, lymph nodes, lymphoid organs. Medical illustration depicting the lymphatic system in a silhouette of a 3 year old child. This system is made up of a network of lymphatic vessels and lymphoid organs. There are two types of lymphoid organs. The primary lymphoid organs are the thymus and bone marrow. Lymphocytes are produced, develop and are selected from primary lymphoid organs. But there are also the secondary lymphoid organs. The secondary lymphoid organs are the spleen, lymph nodes, lymphoid tissues associated with mucous membranes (Peyer's plaque). These secondary lymphoid organs are the site of activation of naive lymphocytes, and therefore the starting point of the adaptive immune response.