Induration is a characteristic feature of delayed-type
hypersensitivity skin reactions and is the usual measure of their intensity. The precise basis of induration has not been established, although activation of the clotting system with consequent
fibrin deposition has been clearly implicated. In this study, two subjects with
congenital afibrinogenemia, a genetic defect in
fibrinogen synthesis, were skin tested with standard microbial
antigens:
streptokinase-
streptodornase, monilia,
mumps, and
tuberculin purified
protein derivative. One positive delayed reaction from each subject was biopsied at 40-48 h and compared with 23 biopsies of similar skin tests in normal volunteers. The eight skin tests in the afibrinogenic subjects lacked induration, although the
erythema was similar in size (10-34 mm in diameter), intensity, and time-course to those in normals. Biopsies from the two strongest reactions from the afibrinogenemic subjects showed a typical perivascular mononuclear infiltrate. No more than traces of
fibrin/
fibrinogen were detected by immunofluorescence, in striking contrast to the abundant
fibrin/
fibrinogen deposition in 23 positive, indurated reactions in normal subjects. These findings indicate that
fibrinogen itself is essential for the development of induration in delayed-type skin reactions in man. As judged by 1-mum sections and fluorescence, this is probably a result of the formation of an extravascular
fibrin gel.