Immunodeficiency can be Primary: Genetically determined, typically manifesting during infancy or childhood Secondary: Acquired, the most common cause of immunodeficiency There are many causes of secondary immunodeficiency, but most immunodeficiencies result from one or more of the following: Systemic disorders (eg, diabetes, undernutrition, HIV infection) Immunosuppressive treatments (eg, cytotoxic chemotherapy, bone marrow ablation before transplantation, radiation therapy) Prolonged serious illness (particularly in critically ill, older, and / or hospitalized patients) Primary immunodeficiencies are classified by the main component of the immune system that is deficient, absent, or defective: Humoral immunity Cellular immunity Combined humoral and cellular immunity Phagocytic cells Complement proteins Manifestations of immunodeficiency Immunodeficiency typically manifests as recurrent infections.