Lewis antigens are expressed on the surface of red blood cells, endothelium, kidney, genitourinary and gastrointestinal epithelium.[3][4][5] Lewis antigens are red blood cell antigens which are not produced by the cell itself. Instead, Lewis antigens are components of exocrine epithelial secretions, and are subsequently adsorbed onto the surface of the red cell. Lewis antibodies are naturally occurring antibodies, almost always IgM type, found almost exclusively in Le(a-b-) individuals. Lewis antibodies may include a mixture of anti-Le(a), anti-Le(b) and anti-Le(a+).