Genetic factors are one of the main determinants of susceptibility to infectious diseases in humans. The same infectious agent may be capable of causing different immune responses in different infected individuals. The outcome of infectious disease may present in varying clinical presentation due to HLA system. This is due to the immune response for disease process is affected by allelic variation, so some individual are more risky for developing the infection and the other may be protected HLA has become an important tool for understanding the pathogenesis of various infectious diseases. in this book try to determine whether women genotype had an effect on susceptibility to toxoplasmosis and its complication like abortion. this book contain explain about HLA system (its types, methods of investigation, HLA associated diseases) and contain information about Toxoplasma gondii. and relationship between HLA class II and Toxoplasma gondii, and between Toxoplasma gondii and some of autoimmune diseases. the target groups were aborted women and non-aborted.