Overlying the heart, the twin lobed thymus consists largely of developing lymphocytes. The thymus gland influences the activities of lymphoctyes in the spleen and lymph glands. The thymus produces a hormone which stimulates antibody production in the lymphoid tissue. Lymph carries white blood cells to this organ, where they multiply and change into special infection-fighting cells. After puberty, the thymus begins to shrink in size. Its role in the early years of life is not fully understood. It is believed it is important in the development of immunity.