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.