Now let's suppose our APC, the antigen presenting cells such as, let's say, a macrophage engulfs that pathogenic antigen takes that pathogenic antigen and places it onto the MHC class two complex. So this is the MHC class two complex and this green part is our antigen. And once the antigen binds onto the MHC class two complex only then can the CD four AlphaBeta T lymphocyte actually approach and bind onto this complex forming an interaction between the APC, the antigen presenting cell and our T lymphocyte. So it turns out that T cells that have CD four glycoproteins can only bind onto the major histocompatibility complex class two. And as we'll see in just a moment, we also have a major histocompatibility complex class one which these glycoproteins cannot actually bind to. Now, the question is what exactly takes place when this binding is initiated?