immune precipitation

[skspoidn at susssys1.reading.ac.uk]

I am trying to Immune precipitate a cellular protein 
(CD4) with Mabs using protein A and S35 metabolically 
labelled cells, and finding that the immune reaction is
 not occuring, although antibodies are being precipitated.
 Is there some aspect of the immune reaction and susequent
 precipitation which is critical in this procedure?

Thanks in advance

Mike poidinger
Dept of Microbiology
University of Reading