Adenoids are the soft tissue mass behind the nasal cavity near the tonsils. As part of the lymphatic system, the adenoids work together with the tonsils to trap germs and prevent them from entering the mouth and nose. From there, the lymphatic system expels them. In young children, large adenoids can make it difficult to breathe through the nose. At around 5 years of age, the adenoids begin to shrink, and in adulthood they almost completely disappear .