A subset of MHC in humans is human leukocyte antigen (HLA), which controls the antigen-presenting system, as part of adaptive immunity against pathogens such as bacteria and viruses. When human cells are infected by a pathogen, some of them can present parts of the pathogen's proteins on their surfaces; this is called "antigen presentation". The infected cells then become targets for types of cytotoxic T-cells, which kill the infected cells so they can be removed from the body.[8]