Well, let's recall the relationship between antibodies and antigens. So remember, antigens are those pathogenic molecules, usually pathogenic proteins that come from foreign pathogenic cells and which eventually end up in the bloodstream or the tissues of our body, while antibodies are those protein molecules that are produced by the immune cells of our body which eventually are complementary and bind to those antigens. And once the antibody binds onto the antigen and forms the antibody antigen complex. Now, what exactly happens following the formation of the antibody antigen complex? So usually once the antibody antigen complex is formed, that's when we activate the classical pathway of the complement system. So what triggers the classical pathway is the formation of the antibody antigen complex.