Palatine tonsils, occasionally called the faucial tonsils, are the tonsils that can be seen on the left and right sides at the back of the throat. Tonsillar (relating to palatine tonsil) B cells can mature to produce all the five major Ig classes. Furthermore, when incubated in vitro with either mitogens or specific antigens, they produce specific antibodies against diphtheria toxoid, poliovirus, Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus, and the lipopolysaccharide of E. coli. Most Immunoglobulin A produced by tonsillar B cells in vitro appears to be 7S monomers, although a significant proportion may be l0S dimeric IgA. In addition to humoral immunity elicited by tonsillar and adenoidal B cells following antigenic stimulation, there is considerable T-cell response in palatine tonsils. Thus, natural infection or intranasal immunization with live, attenuated rubella virus vaccine has been reported to prime tonsillar lymphocytes much better than subcutaneous vaccination. Also, natural infection with varicella zoster virus has been found to stimulate tonsillar lymphocytes better than lymphocytes from peripheral blood. Combined tonsillectomy and adenoidectomy had a profound detrimental effect on the local IgA response in the nasopharyngeal fluid against poliovirus. These immunological observations paralleled the increased incidence of paralytic poliomyelitis after this operation. Thus, it is obvious that the tonsil have an important role to play in the defense of the host against bacterial and viral infections, and the success of regional mucosal immunity induced by intranasal vaccines most likely depends on these immunocompetent tissues in the oropharynx and nasopharynx. Altogether, therefore, several pieces of direct and indirect evidence indicate that the palatine tonsils are continuously engaged in local immune responses to microorganisms. If the tonsillar lymphocytes became overwhelmed with this persistent stimulation they may be unable to respond to other antigens; the immunological response, particularly in recurrent tonsillitis, may then be impaired. Once this immunological impairment occurs, the tonsil is no longer able to function adequately in local protection nor can it appropriately reinforce the secretory immune system of the upper respiratory tract.