The problem of file pollution is prevalent in p2p file sharing system nowadays. The pollution reduces the availability of system sharply. As the same as natural immune system, p2p file sharing system is also highly distributive, self-adaptive and self-organized. Vector space similarity is computed to weight the votes collected from other peers. Self-adaptive reputation threshold scheme is adopted to estimate the file authenticity. Artificial immunology based anti-pollution object reputation mechanism is established to select neighbor peer set and improve the system availability. Simulation results show that the system can distinguish between good and polluted files accurately and thwart the proliferation of pollution in the network with low communication cost.