The adenoids are a mass of soft tissue behind the nasal cavity. Like lymph nodes, adenoids are part of the immune system and are made of the same type of tissue (lymphoid tissue). White blood cells circulate through the adenoids and other lymphoid tissue, reacting to foreign invaders in the body.
We all have adenoids at birth and in childhood, but as we head into adolescence they start to shrink. By adulthood, most people's adenoids have disappeared.