Fab and F(ab')2 are respectively mono and bivalent fragments of immunoglobulins, i.e. including one or two sites for the fixation of the antigen. They are prepared by enzymic digestion (papain or pepsin). They are purified by affinity chromatography and are thus particularly specific.
Fab and F(ab')2 fragments have certain advantages with respect to the entire antibodies in immunohistochemistry, immunocytology and flow cytometry tests.
On the one hand their specificity is increased since they can no longer be bound in a non-specific way by their Fc part as entire Ig’s are.
On the other hand their use also increase the sensitivity of the technique. In fact, their small size enhances their penetration into tissues and cells as well as their availability for the antigens (this is so particularly for Fab).
Finally, conjugate Fab\'s have a label/antigenic ratio that is twice as much compared to antibodies (except fluorescent conjugates).
The F/P (fluorochrome/protein) molar ratio is set between 3 and 7.