Primary immunodeficiencies include a variety of disorders that render patients more susceptible to infections. If left untreated, these infections may be fatal. The disorders constitute a spectrum of more than 80 innate defects in the body's immune system. Primary immunodeficiencies generally are considered to be relatively uncommon. There may be as many as 500,000 cases in the United States, of which about 50,000 cases are diagnosed each year. Common primary immunodeficiencies include disorders of humoral immunity (affecting B-cell differentiation or antibody production), T-cell defects and combined B- and T-cell defects, phagocytic disorders, and complement deficiencies. Major indications of these disorders include multiple infections despite aggressive treatment, infections with unusual or opportunistic organisms, failure to thrive or poor growth, and a positive family history. Early recognition and diagnosis can alter the course of primary immunodeficiencies significantly and have a positive effect on patient outcome.