I am currently isolating exosomes from plasma using size exclusion chromatography (sepharose 2B column). I get a nice separation between vesicles and protein, and a size distribution between 20-200 nm. I end up with a vesicle concentration of 2*10^11 vesicles/ml. However, I am having difficulties getting clear bands on a western blot with commercial antibodies against CD63 and CD81. Could my concentration simply be too low? Recommendation of good commercial antibodies against exosomes would be appreciated.