April 3, 1941.
Dear Enrique:
As a result of conversation with you and Pillemer, it is evident that we are in a position to dictate complement nomenclature, Sherman Act or no Sherman Act.  We have been mulling over things since your stimulating paper was given and are inclined to agree with Pillemer's proposal to use C' as a symbol for complement, C to be reserved for bacterial somatic specific Carbohydrates and S for Soluble Specific Substance, or capsular polysaccharides.
Then erstwhile "midpiece" = C'1
" "endpiece" = C'2
3" component = C'3
and 4" " = C'4
Inactivated complement = iC', small i being less easy to confuse with Roman I.
Inactivated "midpiece" = iC'1
" "endpiece" = iC'2, etc
What I have called complement combining component (or components)", or Cm, would then be C'1, C'1, '4, C'4,1 depending on the outcome of your further studies.
I am particularly anxious to come to an agreement on this because I would like to recall the two quantitative papers I now have in press before they reach the printer, in order to make the above changes in lettering and put in a footnote saying it is in agreement with your group.  In the normal course of events the papers should be finished with the editorial mill in a few days.  Then if, in your papers, the same terminology is used, it should be comprehensible to all and soon become established.  So please let me know as soon as possible.
You and Pillemer contributed greatly to the success of the Conference and it was fine to see you again, too.
Hoping the work will prosper, and with cordial greetings,
Sincerely,
Michael Heidelberger.