Immunoprecipitation analysis of allele-specific recombinant CD11b isoforms. Recombinant forms of CD11b/CD18 were produced in COS cells transfected with either the CD11b R61 isoform (Mart-positive; lanes 2 and 5) or the CD11b H61 isoform (Mart-negative; lanes 3 and 6). After cotransfection with CD18 construct, COS cells were surface labeled with biotin and then lysed. Nontransfected COS cells were used as negative control (lanes 1 and 4). Cell lysates were precipitated with anti-Mart or mAb LPM-19c against CD11b. Immunoprecipitates were analyzed on 7.5% sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) under reduced conditions, blotted, and visualized using streptavidin-horseradish peroxidase and chemiluminescence substrate.