So let's suppose we have some type of pathogen. Let's suppose a bacterial cell, as shown in red. And the macrophage finds our bacterial cell and engulfs that bacterial cell. And so in diagram two we form this phagosome, this vacuole inside our macrophage. And these lysosomes fuse with this vacuum and they begin to break down that bacterial cell. Now, as they break down the bacterial cell, they essentially take some of those pathogenic proteins or antigen and the cell places our antigen onto our MHC class two complex.