Now, generally speaking, the complement system can be broken down into two pathways, into two major mechanisms. We have the classical pathway that we're going to focus on in this lecture and we have another pathway known as the alternative pathway, which we're going to focus on in the next lecture. So what exactly is the classical pathway? 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.