We developed and optimized a cell-based ELISA for specific detection of tumor-reactive antibodies in order to obtain a robust and optimized system for the detection of anti-tumor antibodies from patients ( Figure S1 ). We first evaluated the sensitivity and specificity of an IgG antibody against a melanoma cell surface antigen (HMW-MAA), expressed on A-375 melanoma cells using immunocytochemistry (cytospins) and live cell flow cytometry. An anti-HMW-MAA antibody was observed to bind to A-375 cells, but not melanocytes over a range of concentrations as low as 20 ng/mL using both immunocytochemistry (Figure 1 , A) and flow cytometry (Figure 1, B) . Next, we compared the detection of this antibody bound to melanoma cells in our cell-based ELISA to the above methods.