Progress in immunologic methods and diagnostic assays over the last two decades has led the description of an association between some cases of recurrent fetal loss and recurrent spontaneous abortion of unknown cause to immunological factors. Some causal immunological diseases or dysregulations have been identified in pregnant women. These abnormalities may precede or be triggered by the pregnancy, or occasionally result from an impairment of the mother's allogeneic response to the fetus. In this article we review the principal immunological factors and abnormalities implicated in recurrent pregnancy loss and focusing on those generally accepted by the majority of authors.