title: Emergence and Evolution of Secondary Lymphoid Organs.
abstract: For effective adaptive immunity to foreign antigens (Ag), secondary lymphoid organs (SLO) provide the confined environment in which Ag-restricted lymphocytes, with very low precursor frequencies, interact with Ag on Ag-presenting cells (APC). The spleen is the primordial SLO, arising in conjunction with adaptive immunity in early jawed vertebrates. The spleen, especially the spleen's lymphoid compartment, the white pulp (WP), has undergone numerous modifications over evolutionary time. We describe the progressive advancement of splenic WP complexity, which evolved in parallel with the increasing functionality of adaptive immunity. The Ag-presenting function of follicular dendritic cells (FDC) also likely emerged at the inception of adaptive immunity, and we propose that a single type of hematopoietically derived APC displayed Ag to both T and B cells. A dedicated FDC, derived from a vascular precursor, is a recent evolutionary innovation that likely permitted the robust affinity maturation found in mammals.
authors.affiliation: Department of Microbiology and Immunobiology, Harvard Medical School, Boston, Massachusetts 02115.
authors.forename: Harold R
authors.initials: HR
authors.lastname: Neely
authors.affiliation: Department of Microbiology and Immunology, University of Maryland, Baltimore, Maryland 21201; email: mflajnik@som.umaryland.edu.
authors.forename: Martin F
authors.initials: MF
authors.lastname: Flajnik
mesh_terms: D000818:Animals; D005075:Biological Evolution; D006801:Humans; D008221:Lymphoid Tissue / Q000196:embryology*; D008954:Models, Biological; D014714:Vertebrates / Q000196:embryology
keywords: adaptive immunity; antigen presentation; evolution; spleen; white pulp