Evolution of the immune system

gbeck at Path.SOM.sunysb.edu gbeck at Path.SOM.sunysb.edu
Thu Dec 3 10:37:10 EST 1992


    In answer to Larry Moran's comments (lamoran at gpu.utcs.utoronto.ca), yes, 
it's about time to generate discussion about invertebrate defense systems. 
Just to reply to some specific comments:

>Yes, it would be wonderful to study ancient defense systems but 
>unfortunately they only exist in organisms that are now extinct (by 
>definition). 

    An advantage of examining the more ancient cellular defense response, 
rather than the humoral system, may be that it will allow us to jump over 
gaps in the fossil record.  The phagocyte, which is present in every extant 
animal and probably was found in a majority of extinct species as well, 
provides a handle for tracing the development of a major component of the 
immune system through evolutionary (geologic) time.  This is in contrast to 
studies of more recently developed immune mechanisms (i.e., rearranging 
immunoglobulins or B lymphocytes) that cannot be traced directly because the 
species in which they evolved and developed are extinct.  Since phagocytosis 
is as old as animal life itself, it is likely that the armamentarium of the 
phagocyte has ancient origins as well.

>what I object to is the implication that human defense mechanisms are 
>advanced and modern while others are ancient and primitive. We would all do 
>well to remember that evolution is best represented by a branching bush and 
>not by a ladder with vertebrates on the top.

    The imprecise word *primitive* is used deliberately in our discussions 
because of our ignorance about the molecular details of invertebrate host 
defenses.

>Similarly,the mechansisms employed by plants, arthropods and other 
>invertebrates have evolved over a period of hundreds millions of years from 
>the time of the last common ancestor of these organisms and vertebrates. 
>Given that there is such variety within vertebrates and that the immune 
>systems are so sloppy it doesn't seem very likely that there is much of an 
>evolutionary history to detect. Very few of the molecules of the vertebrate 
>immune system have an evolutionary history that predates the origin of 
>chordates, this suggests that most of the vertebrate immune system is 
>fairly new on the scene.

    We should not think of the vertebrate immune system as just a bundle of 
molecules. The humoral system is a recent player, but the cellular defense 
systems have been working well for a very long time. Since invertebrates 
represent >95% of all species there is something to be said for their 
cellular based defensive systems. 
    We need to keep these discussions going.  We need to establish a common 
language for describing the host defense systems which have participated in 
selecting the species, both vertebrate and invertebrate, which are found on 
the earth today. Major misunderstandings about the evolution of immune 
function clearly persist and continue to confuse the interpretation of 
results in higher vertebrate systems.

Thanks
Greg Beck & Gail Habicht
|**********************************************************************|
|*  Gregory Beck																				  		GBeck at path.som.SUNYSB.edu     *|
|*  Department of Pathology                                           *| 
|*  State University of New York at Stony Brook                       *|
|*  Stony Brook, NY 11794-8691  USA                                   *|
|*  Telephone 516-444-3030                                            *|
|*  FAX 516-444-3424                                                  *|
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