Re: Why is checking cell viability important?

Checking the cell viability is important for lots of reasons.  Mainly 
because if you count 100 cells and only have 50% viability when you go to 
use them you may find that your experiment didn't work terribly well.  
Probably because you only had half the number of viable cells that you 
thought you did!  In addition, there are certain things that can be 
recognised on dead cells and depending on how they died (necrosis or 
apoptosis ie the cell bursts or programmed cell death) and these could 
also affect the experiment.

If I get your question correctly, you want to know why it's important to 
check the viability when using immunological techniques, I'd say that 
it's important for the above reason as well as the uptake of 
fluorochromes.  When you phenotype cells by immunological methods eg flow 
cytometry, dead cells will take up the fluorochromes that are used to 
label the antibodies used in phenotyping.  For example, if you wanted to 
phenotype CD4 and CD8 expression, dead cells may give false positives.  
They don't maintain membrane integrity and dyes and fluorochromes can 
enter into the cell making it red or green or both.  The same would apply 
to other fluorescent techniques. Other dyes, such as propidium iodide 
(PI), can be used in flow cytometry to exlude dead cells.  Dead cells 
allow PI to enter into the cells and the PI binds to DNA.  These cells 
would appear red-ish by flow cytometry.  So you could exlude all red-ish 
cells when phenotyping.

On top of that, particularly if you wanted to phenotype cells using a 
flow cytometer, dead cells may have a different size and granularity from 
live cells.  As flow cytometry also relies on looking at the size of 
cells, recognising dead cells is important.  The cytometrist will 
probably be able to adjust the machine to exclude any dead cells 
(although this can be tough).
  
Hope that answers your question.