HUMORAL AND CELL-MEDIATED DEFICIENCIES
DISEASE	IMMUNE-SYSTEM DEFICIENCY	POSSIBLE MECHANISM
HUMORAL DEFICIENCIES
X-linked agammaglobulinemia (XLA)	Reduction in B-cell count Absence of immunoglobulins	Block in maturation of pre-B cells due to defective V-D-J gene arrangement
X-linked hyper-IgM (XHM) syndrome	Low levels of IgG and IgA Very high levels of IgM	Defect in class switching
Common variable hypogammglobulinemia (CVH)	Decreased plasma-cell levels but usually normal B-cell levels Variable reduction in secreted Ig of all isotopes	Defective differentiation of B cells to plasma cells due to defective processing of Ig transcipts, lack of cytokine receptors on B-cells, or abnormal T-cell response
Selective immunoglobulin deficiencies	Decreased levels of one or more Ig isotopes	Defect in maturation of plasma cells or necessary T-cell-derived cytokines
CELL-MEDIATED DEFICIENCIES
DiGeorge syndrome	Decreased T-cell counts	Lack of T-cell maturation due to absence of thymus
Nude mice	Decreased T-cell counts	Lack of T-cell maturation due to absence of thymus