The effect of protein concentration on background and detection limits. Six antigen mixes were added to fetal calf serum (FCS) solutions to produce a 10-fold and 100-fold increase in total protein concentration and a corresponding decrease in partial concentration of the antigens. After labeling, the mixes were analyzed with antibody microarrays. The images at the top of the figure present a detail from one of the arrays in each set. The median log10 of the Cy5/Cy3 fluorescence ratio is plotted as a function of the concentration ratio of the cognate antigen of the three indicated antibodies. The dashed line represents the log10 of the true ratio of antigen concentrations in the Cy5-labeled and Cy3-labeled solutions.