These antibody coated neutrophils are removed from circulation by the spleen where they are phagocytized by macrophages. AIN is classified as primary or secondary cases. Primary AIN is not associated with other immunological or hematological abnormalities. Primary autoimmune cases are observed most frequently in newborns with an incidence of 1/100,000. Primary AIN is usually diagnosed during the first few months (5-15 months) with severe neutropenia seen at presentation with Absolute Neutrophil Counts (ANC) between 500-1500 neutrophils/µL. The clinical course is usually benign or at any rate self-limiting, with a tendency to resolve spontaneously by 2 or 3 years of age. Severe infection such as pneumonia, sepsis, and meningitis are observed in approximately 12% of patients.
Chronic idiopathic neutropenia in the adult differs from the neonatal autoimmune form in that it appears much later life with the incidence higher among females than males (70% to 30% respectively). The adult form exhibits little tendency toward spontaneous remission. It remains clinically benign with only 35% of cases positive for neutrophil autoantibodies.
Secondary AIN is associated with other autoimmune disorders such as rheumatoid arthritis (RA), Felty’s syndrome or systemic lupus erythematosus (SLE). Secondary AIN is most commonly observed in individuals between 40 to 80 years of age. Severe or life-threatening infections are not common in patients with secondary AIN, however a pattern of recurrent cutaneous and upper and lower respiratory tract infections was noted.