The immune system is necessary to defend the body against foreign antigens, as well as to preserve the integrity of the body and maintain homeostasis. To adequately fulfill these roles, the normal function of its executive arms, ie humoral and cellular immunity, is necessary. One of the main features of an immune reaction is the recognition of one's own from another's, that is, the recognition of foreign antigens. For the immune system to be able to distinguish others from its own, it is necessary to adequately regulate the immune response and develop immune tolerance to one's tissues and organs. Numerous pathogenetic mechanisms lead to the abolition of immune tolerance and the consequent loss of the ability to recognize one's antigens, leading to the activation of immune system executive mechanisms and the consequent development of autoimmunity, which may or may not be clinically manifest. Selective immunoglobulin A (IgA) deficiency is a disorder of class A immunoglobulin production and is etiologically and pathogenetically associated with several autoimmune diseases in children and adults, including type 1 diabetes. Autoimmune type 1 diabetes mellitus is caused by destruction of islet Langerhans beta cells and consequent insulin deficiency. The clinical picture of selective IgA deficiency includes an increased incidence of infections, allergies and atopic disorders, and associated autoimmune diseases. Selective IgA deficiency and type 1 diabetes mellitus share the same genetic predisposition and pathophysiological mechanism of development and have a higher frequency of concomitant occurrence. Although there is currently no indication that the presence of IgA deficiency in patients with type 1 diabetes mellitus affects morbidity and metabolic regulation of the disease, long-term follow-up of more patients is needed to assess the potential negative effect of selective IgA deficiency.