But these constant regions do change when we go from one type to a different type. In fact, these constant regions determine the type of class that we're dealing with and they actually determine the mechanism that our antibody uses to basically fight off infections and fight off these pathogenic invasions. And on top of that, it's these constant regions, the bottom portion of these constant regions that actually bind onto the cell membranes of immune cells and other cells of our body. So now that we know what the structure of the antibody looks like, let's discuss what the mechanism is by which the antibody actually elicits a defensive response by our immune system. So antibodies can either be found bound onto the membrane of immune cells or they can float around as free antibodies within the blood or lymph of our body. And either way, when the antibody locates and binds onto that specific antigen for which it was actually built for, they form the antibody antigen complex.