Fluorescent dextrans are retained well by neurons for at least 30
days after labeling (at least in larval zebrafish neurons in vivo).
However, one presumably would have to inject them into lymphocytes.
(We retrogradely label neurons by disrupting their axons far from
their cell bodies).  I think these compounds are fairly "inert"--
we've used calcium green dextran and texas red dextran.  While texas
red dextran seems initially brighter, it bleaches like mad in our
hands.  Calcium green dextran handles repetitive imaging far more
stably (and non-toxicly) in our prep, but is a calcium buffer and
would dampen calcium transients in these cells.
   I haven't had much experience with latex beads-- but I suspect
they might be better.  All of this is available from Molecular
Probes, Eugene OR.

Might be worth a try,
Don O'Malley
Northeastern University