An immunodeficiency can cause frequent and recurrent infections of varying severity. The disorder may be due to underlying medical conditions, or treatments for these conditions. This is termed secondary immunodeficiency and is a much more common reason for recurrent infections in adults. A less common cause is primary (congenital) immunodeficiency, which typically presents in infancy or early childhood.
Primary immunodeficiency typically denotes inherited abnormalities of the immune system. Primary immunodeficiency can affect T cells, B cells, phagocytic cells or the complement system.
Most primary immune deficiencies are inherited diseases; examples include X-linked agammaglobulinemia (XLA) and severe combined immunodeficiency (SCID), and this appears to run in families. Other primary immune deficiencies, such as common variable immunodeficiency (CVID), appear less obviously inherited. However, the causes of the defects are unknown, and genetic factors cannot be ruled out.
Secondary immunodeficiencies occur when the function of the immune system is disrupted secondary to an underlying disease state, medications or surgical and other medical procedures. Radiation, chemotherapy, burns and chronic illnesses and malignancies can causes secondary immunodeficiency. Infections with the Human Immunodeficiency Virus (HIV) cause a secondary immunodeficiency known as Acquired Immune Deficiency Syndrome (AIDS). In leukemia and multiple myeloma, cancerous immune cells crowd out the normal stem cells of the bone marrow. These abnormal cells reduce the number of B cells and lead to hypogammaglobinemia, another type of secondary immune deficiency.