The present invention relates to the treatment and prevention of cancer, viral infections and microbial infections by the administration of anti-C3b(i) antibodies. The present invention also relates to methods of treating and preventing cancer, viral infection, or microbial infection in an animal comprising administering to said animal IgG antibodies, IgM antibodies and/or complement components in combination with antibodies specific for C3b(i). The present invention also relates methods of treating and preventing cancer, viral infection or microbial infection in an animal comprising administrating said animal antibodies that immunospecifically bind to one or more cancer cell antigens, viral antigens or microbial antigens, respectively, in combination with antibodies immunospecific for C3b(i). The present invention further relates to the detection, imaging, diagnosis and monitoring of cancer utilizing C3b(i) specific antibodies.