Summary. MYD88 deficiency is a rare primary immunodeficiency characterized by an increased susceptibility to certain types of bacterial infections. Myeloid differentiation primary response 88 (MYD88) is a protein that, in humans, is encoded by the MYD88 gene.The MyD88 protein is part of a signaling pathway that is involved in early recognition of pathogens and the initiation of inflammation to fight infection.