Individuals with selective IgM deficiency may be asymptomatic or symptomatic. Of those symptomatic, approximately 80% present with predominant bacterial infections. Among infections, the most common are chronic sinusitis, upper respiratory tract infections, bronchitis, and pneumonia. Occasionally cellulitis, sepsis, and meningitis have been observed. Almost 40% of individuals with Selective IgM Deficiency have allergic diseases, including hay fever and asthma. Autoimmune diseases are observed in about one-third of all individuals with selective IgM deficiency. Autoimmune diseases are more common in adults than children with selective IgM deficiency. Some common autoimmune diseases associated with selective IgM deficiency are systemic lupus erythematosus, rheumatoid arthritis, and autoimmune thrombocytopenia. Autoimmune diseases of endocrine glands, like Hashimoto’s thyroiditis and Addison’s disease, have also been seen in selective IgM deficiency.