Antigenic and ICAM-1 binding properties of shed LFA-1 in blister fluid. (A) An LFA-1–specific ELISA assay was developed that used a variety of CD11a and CD18 antibodies in the capture step, in each case detected with biotinylated anti-CD11a antibody 38. Results represent the mean ± SD concentration of soluble LFA-1 (ng/mL), calibrated using recombinant LFA-1 as a standard. (B) The LFA-1 ELISA was modified with ICAM-1–Fc (5 domain) in the capture step to examine binding by soluble LFA-1 in blister fluid to solid-phase ICAM-1, in the presence and absence of blocking anti-CD18 antibody 6.5E. Recombinant LFA-1–Fc at 5 ng/mL was used as a positive control. Results represent mean ± SD absorbance readings at 405 nm, detected using either biotinylated mAb 38 or IgG_2a control antibody.