Diabetes mellitus is a group of metabolic diseases caused by a combination of insulin resistance and impaired insulin secretion by pancreatic β cell. In 2007, 246 million people (roughly 6%) were affected by diabetes worldwide and it is estimated that this will increase to 380 million in 2025. Diabetes is associated with several long-term complications such as cardiovascular disease, nephropathy, retinopathy, neuropathy and some oral complications. In addition, Diabetes mellitus causes an increased risk of morbidity because of infection disease. It seems that, the increased frequency of infections associated with Immunoglobulin-A (IgA) deficiency in these patients. Therefore a low IgA secretion rate is suspected to be one of these mechanisms. Moreover, it shows a main antiviral activity by neutralizing toxins and viruses. It by inhibiting the attachment and replication of pathogenic microorganisms prevents colonization of these pathogens.Therefore, it acts as a first line of defense against pathogens and early detection of immunoglobulin A deficiency in diabetic patients can prevent the vicious cycle of recurrent infections and reduces risk for morbidity and metabolic decompensation. Moreover, the Salivary-IgA is the widespread immunoglobulin in mixed saliva and is assumed to be an important factor for adaptive immunity in the oral cavity. Therefore, according to these studies, Immunoglobulin A, its mechanism, IgA deficiency and diabetes and its relation with oral complications are explained in this paper.