Severe combined immune deficiency (SCID) is a life-threatening primary immunodeficiency (PI), with a combined absence of T cell and B cell function. There are at least 20 different genetic variants that can cause SCID.

Selective IgA deficiency is a primary immunodeficiency characterized by an undetectable level of immunoglobulin A (IgA) in the blood and secretions but no other immunoglobulin deficiencies.

Leukocyte adhesion deficiency (LAD) is a primary immunodeficiency that causes individuals to be abnormally susceptible to developing frequent soft-tissue infections, gum inflammation, and tooth loss.

Patients with persistently low levels of one or two IgG subclasses and a normal total IgG level have a selective IgG subclass deficiency.

Hemophagocytic lymphohistiocytosis (HLH) occurs when histiocytes and lymphocytes become overactive and attack the body rather than just microorganisms. Familial HLH is a type of PI.