Andrea,
                this is a barrel of worms.
The sensitivity of  FACS can be interpreted several ways. The most important
thing is the number of labeled cells that is sorted and the skill of the
operator and the evaluator.  Many of the studies deal with lymphocyte sub
populations where large numbers of cells are available. If however you are
dealing with cells separated from tissues and the number of labeled cells is
below a certain percentage then you will probably have a lot of difficulty in
separating and identifying the cells unless you have a large amount of
material..
As regards sensitivity I assume that we are talking about the lowest limit of
detection. Of equal importance is the selectivity (ability to specifically
detect when in a mixture), precision (reproducibility of a method), and
accuracy (closeness of the test results to the true value).

If  we are talking about light microscopy.
Detection of label on cells by IHC will depend on several factors.
1.    Your optical system for examining the cell label. Epi illumination is
generally better than transmitted, using appropriate excitation and barrier
filters for fluorescence, type of optics etc.
2.    The type of label, fluorescent, enzyme etc.
In general, fluorescent labels can be detected at a much lower level but are
not always so easy to specifically localize.
3.    Your tissue preparation
        Fixation and processing with possibility of  induced - and
auto-fluorescence
        specificity of your labeling
        amount of label
        counterstain
        thickness of sections can  play a role especially if using non
fluorescent markers
        mountant

Hope I haven't made this seem an impossible task.
EM is a different barrel of worms
Barry

AndreaH@imclone.com wrote:

> Does anyone know of a reference that describes and compares the sensitivity
> limits/levels of FACS and/or IHC.  In addition is anyone aware of a
> reference or does anyone know how many receptors there must be on a cell
> before it can be detected by IHC??
>
> Thank you in advance.
>
> Curiously,
> Andrea T. Hooper
>
> ==========================
> ImClone Systems Incorporated
> E-mail: andreah@imclone.com
> ==========================