Immuno is short for immunohistochemistry. Its a type of experiment that we do on cells or tissues.
We take the cells/tissue we are interested in and fix them in formaldehyde (this pickles them). We can then make really thin slices through the tissue so that we have a cross section of that tissue. This is really useful because we can then use stains to see what the tissue looks like and compare it to other diseased or healthy tissue and see how its changed.
We can then also look for particular cells or proteins within the cross section and see if there is more or less of them and also where they are in the tissue.
Its a really useful technique and is used my lots of scientists. Its also used in hospitals to diagnose some diseases which they to this by taking biopsies (a small bit of tissue).