So let's suppose we have the CD four T cell. And what that basically means is it contains a T cell receptor that contains an alpha unit shown in purple and the beta unit shown in red. And it also contains a special glycoprotein that extends from the cell membrane and that is shown in blue. 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.