Collagen fiber is the most widely distributed and abundant fiber in connective tissue. It is widely distributed in various organs. Type I collagen fibers are mainly bone， skin and tendon; Type Ⅱ collagen fibers are mainly cartilage collagen; Type Ⅲ collagen fibers are mainly found in embryonic tissues， adult blood vessels and gastrointestinal tract; Type Ⅳ collagen fibers are mainly in the basement membrane. Aniline blue， as a macromolecular acidic dye， is easily bound to the alkaline groups in collagen molecules and adsorbs firmly.

Modified Aniline Blue Stain Kit(PAB Method) is mainly composed of Iron Hematoxylin Stain， PAB Staining Solution and Weak Acid Solution， without picric acid. It is mainly used in the study of collagen fiber abnormality or fiber proliferation in various tissue diseases. Under the ordinary optical microscope， the collagen fibers of heart and blood vessels and other tissues are dyed blue to blue green. Immunohistochemical technique can also display type Ⅰ and Ⅲ collagen fibers， but the antibodies used are expensive and time-consuming， while PAB staining reagent is cheap and easy to operate.