I performed a WB using plasma rats and monkeys samples with anti ubiquitin K-48 antibody. In every sample the antibody binds something and it appears a specific band that seems to be 71kDa. When I used anti-ubiquitin unspecific antibody(just to see if I get the same results) same bands appears. It cannot be ubiquitination because the molecular weight. How can I explain the both antibodies bind something that I dont know?