Blocking Fc receptors - summary

An interesting question. 

I haven’t evaluated, but first thought would be that mouse blocking would work well; after all, that is exactly the binding that you need to block.  Further, there would be the least likelihood of mouse Ig blocking containing any anti-mouse reactivity against the mouse monoclonal reagents, whereas blocking with human IgG may contain some anti-mouse reactivity, as can be a problem in some immunoassays.  Jackson Labs has probably addressed this matter in one of their preps. (A caveat is if the mouse monoclonal reagent is of a class which makes up a small fraction of normal mouse serum Ig, and thus the blocking concentration is lower that expected.)  

 Hello,

 you would be using Human IgG for iv therapy which is commonly sold as 
50mg IgG per ml. This solution is sterile, stable (store at 4°C!!; no
(!) freezing) for prolonged periods, tested for infectious agents and
rather inexpensive (considering how long a vial will last). You may ask
the manufacturer also for lots that have just expired and cannot be
used for therapy any more. This is usually given away free and as good
as the "real thing".

 For use, make stock solutions by diluting with your preferred wash IF
wash buffer to 10mg/IgG per ml. This stock may be stored frozen. 

 For blocking, use one drop (!) per assay before adding the test
antibody. If you prefer exact dosage take 20ul (but only difference
being that it takes more time). No preincubation or washing required.
Just mix.

 Same approach with indirect immunofluorescence - BUT in such case your
second step reagents MUST be absorbed with human IgG.

 This protocol works with almost any species. It fails in extremely (!)
rare instances when one encounters cells carrying IgM-Fc Receptors
where you need human IgM for blockage.

 Hope this helps!

 With best wishes

I would think, that all things you suggest would work.
We find as best (and of course cheapest) solution, to block the human cells
with mouse serum. We can get it from our animal facility for free or make it
ourselfs (Old mice and mice with wrong genotype are always available).
Mouse serum contains a lot of unlabeled IgG, which should block all kind of
unwanted binding sites on your cells (not only Fc-receptors).
If you use solely mouse mAb, than only mouse antibodies carry your dyes and
therefore can contribute to your background.

In our hands, this worked fine.

Happy blocking

PS: We used about 5-10% mouse serum in PBS and block 10-15 min at 4°C (on
ice, to be more correct).

Back in the old days we always used expired human AB serum from the blood
bank.
We couldn't use mouse serum even if we had any because we were doing indirect
staining. Presumably human Ig would fit the Fc receptors better than mouse
which would make it more effective. You do of course have to heat inactivate it
to prevent complement lysis of the bound cells. 60 degrees C for 30 mins. We
would then filter though a syringe filter and freeze it. We preincubated  with
5% serum for 15mins before adding the antibodies. Seemed to work pretty well.

Best