FITC (Fluorescein Isothiocyanate), is one of the green fluorescein derivatives widely used in biology. FITC has the characteristics of high absorptivity and excellent fluorescence quantum yield. The isothiocyanate group of FITC can be combined with amino, sulfhydryl, imidazole, tyrosyl, carbonyl and other groups on the protein, so as to achieve protein labeling including antibodies and lectins. In addition to its use as a protein marker, FITC can also be used as a fluorescent protein tracer to rapidly identify pathogens by labeling antibodies, or for microsequencing of proteins and peptides (HPLC). The maximum excitation wavelength of FITC is 494 nm. Once excited, it fluoresces yellow-green at a maximum emission wavelength of 520 nm. In addition, FITC is also a hapten that can induce contact hypersensitivity (CHS) and induce an atopic dermatitis model .