The antibodies have epitopes that bind to the paratropes present on the antigens. When an antibody is made specifically to target a particular kind of antigen (say A), its epitope will be a structural compliment to the paratrope. When an antibody encounters two different antigens (say A and B) with structurally similar paratropes, it will bind specifically to A and non-specifically to B, as the antibody is not made in response to antigen B. This non-specific binding is referred to as blocking.
