Primary immunodeficiency (PID) is an intrinsic defect of the immune system. Patients with PID have increased susceptibility to recurrent and persistent infections, and they may also have autoimmune and cancer-related symptoms. PIDs are mainly rare hereditary disorders of the immune system which often have serious consequences. Infections are the hallmarks of PIDs. Clinical descriptions have already been made for more than 200 PIDs, for which approximately 170 genetic aetiologies have been described. There are several different mechanisms behind PIDs. Some PIDs affect T- or B-cell functions and when both cell types are affected lead to severe combined immunodeficiencies (SCIDs). Other PIDs affect major histocompatibility complex, antibody production, lymphocyte apoptosis, phagocytosis, complement cascade or the innate immune system.

Key Concepts:

• Adaptive immunity is a specific and long-lasting response of lymphocytes to antigens.
  
• Combined immunodeficiency affects both B- and T cells.
  
• Lymphocytes are white blood cells including B- and T cells.
  
• Signal transduction is a process by which a cell converts one kind of signal or stimulus into another. In a signalling cascade an extracellular signal interacts with a cell surface receptor, which leads to a change in the level of a second messenger and ultimately a change in the cells functioning.