I'm trying to develop a direct ELISA against proteins from cell culture supernatent. When I plate the super and check for plating using an antibody to a protein I know to be present in the media, I see an effect where I get an increase in signal with an increase in dilution. The more dilute I get the higher the signal. The signal eventually plateaus and comes down to background with further dilutions. Is this a common phenomenon? How is it explained?