And some type of protein that the cell produces inside our cell, it brings out and it binds onto this region right here. So when a leukocyte essentially approaches this healthy cell and it recognizes this peptide as a normal peptide, as a normal self antigen, so we call this a self antigen, which basically means that it's an antigen, it's a protein that is formed by this cell. So when the leukocide recognizes a self antigen, it basically leaves that cell alone. It knows that that is a healthy cell. Now, what happens if a cell is actually infected by some type of pathogen. For example, a virus.