Meaning of "Antibody is 50% w/w pure of interfering proteins"
Patents are not the best source for reliable information, and obviously are not that readable. In this case they mean that of the purified product (usually that what comes of a protein A column I think), at least 50% of the weight is antibody. Or less (silly patents). 

Commercial purification of antibodies (and proteins in general) is usually not that thorough. Most of the times what you'll get is just culture supernatant with cells removed, maybe some ammonium sulfate precipitation fraction or if it's thermostable just the proteins that are left after the production-solution is heated.