doi:10.1016/j.it.2004.10.001 
Copyright © 2004 Elsevier Ltd All rights reserved.
Evolution of innate and adaptive immunity: can we draw a line?
Martin F. Flajnika, *, 
and Louis Du Pasquierb,
aDepartment of Microbiology and Immunology, University of Maryland at Baltimore School of Medicine, Baltimore, MD 21201-1559, USA
bInstitute of Zoology, University of Basel, Rheinsprung 9, CH-4051, Basel, Switzerland
Available online 14 October 2004.
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Several recent findings in the field of comparative immunology have reinforced the importance of examining the molecular and functional features of immune systems in a variety of organisms. Particularly exciting are the discoveries of a new gene rearrangement mechanism in lampreys and a somatic diversification of mollusk immune genes. These immune features being found in animals previously believed only to have innate immunity, as well as the flood of information on immune genes, molecules and mechanisms in many different creatures, have prompted us to revisit the artificial dichotomy between adaptive and innate immune systems. Although we draw no startling conclusions, we hope to encourage different thought patterns when viewing immune systems.
Figure 1. Immune molecules and mechanisms found throughout (primarily) the animal kingdom. Red indicates molecules or mechanisms involved in innate immunity. Blue indicates molecules whose genes diversify somatically and are involved (or believed to be involved) in adaptive immunity. Orange indicates somatic mechanisms at the DNA or RNA level to generate or maximize immune diversity (underlined taxa indicate that representatives in these groups have diversification mechanisms of immune genes). Green indicates conserved signaling pathways. Black indicates that genes have been found in linkage groups similar to those in the jawed vertebrate MHC. Abbreviations and short descriptions of each category are all found in Table 1. Details for each of these molecules and mechanisms can be found in Refs 2, 3, 4, 7, 9, 10, 14, 19, 21, 23 and 24, which are all recent reviews on the subject. Abbreviation: C. elegans, Caenorhabditis elegans.
Table 1.
Immune defense molecules and functions found throughout the animal kingdoma
a Note that not all of these features are found in all organisms.
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