The palatine tonsils and nasopharyngeal adenoids represent the predominant immunocompetent tissue of the upper respiratory tract. Its major function is as a first line of defense against viral, bacterial, and food antigens that enter the upper aerodigestive system. Another major function of the tonsils and adenoids is to supply the local mucosal immune system of the upper respiratory tract with dimeric IgA-producing B cells. Secretory IgA has particular hydrophilic properties and is capable of preventing adsorption and penetration of bacteria and/or viruses into the upper respiratory tract mucosa. In addition, the role of the indigenous flora of the upper respiratory tract, particularly the viridans streptococci has been emphasized as providing a valuable source of bacterial interference to the colonization of potential pathogens.