The presence of HLA B27 is associated with an increased incidence of several autoimmune diseases including uveitis. Macrophages and dendritic cells pick up antigens - foreign proteins present in the blood - on their HLA molecules and take them over to T-helper cells which then start an immune response to clear them from the body. (This is why macrophages are called antigen presenting cells.) If these antigen presenting cells pick up proteins manufactured in the body (as opposed to a  protein from a virus) T-helper cells will start manufacture an immune response targeting the body (!). This is what occurs in an auto-immune disease. 

HLA-B27 refers to a specfic variant of HLA molecules that only some people carry. When its present those APC's don't seem to be functioning normally giving those people an increased incidence of auto-immune disease. What exactly's happening, however, is unclear. Many people have HLA-B27 molecules and don't have auto-immune diseases. This means there are more factors involved that have not been uncovered yet.  Its a piece of the puzzle but not a defining one. 

http://emedicine.medscape.com/article/1201027-overview