Lymphangiomas are bumps in the skin filled with a clear or blood-tinged fluid. They are caused by overgrown lymph vessels. Lymph vessels Overview of the Lymphatic System The lymphatic system is a vital part of the immune system. It includes organs such as the thymus, bone marrow, spleen, tonsils, appendix, and Peyer patches in the small intestine that produce... read more transport lymph throughout the body. Lymph is a clear fluid containing white blood cells that help the body fight infection.