COMBINED IMMUNODEFICIENCIES
DISEASE	IMMUNE-SYSTEM DEFICIENCY	POSSIBLE MECHANISM
HUMORAL DEFICIENCIES
Reticular dysgenesis	Decreased numbers of all cells of lymphoid and myeloid lineages Absence of immunoglobulins	Defective maturation of hematopoietic stem cells
Bare-lymphocycte syndrome	Some reduction in CDT4+ T-cell counts Reduced B-cell and Tc-cell activation Decreased TDTH-cell activity	Failure to express class I and/or class II MHC molecules on cells
Severe combined immunodeficiency disease (SCID)   X-linked SCID   ADA-deficiency SCID   PNP-deficiency SCID   CB-17 SCID mouse	Marked reduction in T- and B-cell counts in all forms	Various mechanisms Defective T- and B-cell maturation Defective T- and B-cell maturation Selective killing of lymphocyctes by metabolites that accumulate in absence of adenosine deaminase (ADA) Selective killing of lymphocyctes by metabolites that accumulate in absence of purine nucleoside phosphorylase(PNP) Aberrant D-J joining of Ig heaby- chain and TCR b- and g-chain gene segments.
Wiskott-Aldrich syndrome (WAS)	Low levels of IgM Elevated levels of IgA and IgE Abnormal T-cell function with progressive dysfunction	Defective glycosylation of membrane glycoproteins (CD43) on lymphocytes