Dr. Colin R.A. Hewitt crah1 at
Tue Feb 1 04:19:38 EST 1994

Date:          31 Jan 1994 23:33:45 GMT
From:          trenno at (Toufic Renno)
Reply-to:      trenno at (Toufic Renno)
To:            "bionet.immunology mail newsgroup" <bionet-news at>
Subject:       Re: Definition of POSITIVE SELECTION

trenno at (Toufic Renno) writes:


>This is even more true for lymphocytes: since negative selection does not
>affect lymphoid progenitors, more autorearective lymphocytes will be
>generated AT THE SAME RATE AS BEFORE. The selective process just has to be
>active all the time. This is especially true for B cells that are generated
>throughout life.  Less T cells will be generated as the thymus eventually 
involutes, but this in no way is a result of selection.  
>Bottom line: the peceived decrease in negatively selectable lymphocytes is but
>an optical illusion. 
>Knock three times on the ceiling if you agree.  Twice on the pipe if the answer
>is no.
>Toufic Renno

I'm afraid it's got to be twice on the pipe. 

Involution of the thymus doesn't stop the generation of T cells that need "selecting". 
This gets back to a discussion we had a few weeks ago. Namely: in the absence of a 
fully functional thymus, where do all the new thymocytes (which are generated at the 
same rate as before) get selected. This led to speculation about the thymic remnant 
being active, and the possibility of peripheral mechanisms taking over but I still feel  
the question remains open.

Dr. Colin R.A. Hewitt
University of Leicester/MRC
Centre for Mechanisms of Human Toxicity
Hodgkin Building
PO Box 138
Lancaster Rd

Phone +44 (0)533 525587
Fax +44 (0)533 525616
E-Mail crah1 at

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