Lymphoid organ development: from ontogeny to neogenesis.

The development of lymphoid organs can be viewed as a continuum. At one end are the 'canonical' secondary lymphoid organs, including lymph nodes and spleen; at the other end are 'ectopic' or tertiary lymphoid organs, which are cellular accumulations arising during chronic inflammation by the process of lymphoid neogenesis. Secondary lymphoid organs are genetically 'preprogrammed' and 'prepatterned' during ontogeny, whereas tertiary lymphoid organs arise under environmental influences and are not restricted to specific developmental 'windows' or anatomic locations. Between these two boundaries are other types of lymphoid tissues that are less developmentally but more environmentally regulated, such as Peyer's patches, nasal-associated lymphoid tissue, bronchial-associated lymphoid tissue and inducible bronchial-associated lymphoid tissue. Their regulation, functions and potential effects are discussed here.