Immunofluorescence

Immunofluorescence is a way of seeing, inside a small piece of tissue, where a particular molecule is. It takes advantage of the fact that an antibody binds only to its target, the antigen, and then tags that antibody with a substance that glows under a certain light. It is a spotlight that, in a group photo, lights up only the person you are looking for.

In the direct form, the technique is applied to the patient's own tissue. When the body makes antibodies against itself, those autoantibodies stay stuck to the tissue, and the fluorescent antibody applied on top reveals them.

The molecules usually tracked down are the immunoglobulins IgG, IgM and IgA, which are classes of antibody, and C3, a complement protein. The antibody that hunts for them is tagged with fluorescein.

What matters next is not just that it glows, but where and in what pattern. In bullous pemphigoid, a disease that raises blisters on the skin, the glow appears as a continuous line at the basement membrane, the border between the epidermis and what lies beneath.

And it is not useful only on a tissue section. In flow cytometry, immunofluorescence analysis is the standard for counting CD4 lymphocytes.