The B-cell development in tonsillar lymphoid follicles.

The palatine tonsils, and particularly the nasopharyngeal tonsil (adenoid), may be functionally comparable to nasal-associated lymphoid tissue in rodents. Primary follicles occur in human tonsils at 16 weeks' gestation, and formation of germinal centres (GC) takes place shortly after birth. The GC arise in T-cell-dependent B-cell responses and are associated with: i) clonal expansion; ii) somatic hypermutation in immunoglobulin variable (Ig V)-region genes; iii) positive selection of B cells based on affinity for antigen; iv) differentiation to memory B cells and plasma cells; and v) induction of the J-chain gene. The follicular dendritic cells (FDC) of GC retain native antigen which stimulates growth of B-cell blasts and hypermutation of their Ig V genes. The resulting centrocytes die by apoptosis unless they are selected by antigen. Cognate interaction between CD4+ helper T cells and B cells is important to promote downstream switching of the heavy chain constant (CH) genes. In human tonsillar GC this process normally gives rise mainly to IgG (55%-72%) and IgA (13%-18%) immunocytes, both isotypes normally being partially associated with J-chain expression (36% and 29%, respectively). Because J chain is a key peptide in secretory IgA, tonsillar GC may contribute precursor cells to mucosal effector sites. Thus, mucosal immunity can be induced in the airways by nasal immunization, and the level of nasopharyngeal and salivary secretory IgA is decreased after adenotonsillectomy.