T cell deficiency

T cell deficiency is a deficiency of T cells, either caused by lymphocytopenia of T cells or by decreased function of individual T cells. It causes an immunodeficiency of cell-mediated immunity.

Classifications of causes

By complete versus partial deficiency

Complete insufficiency of T cell function can result from hereditary conditions (also called primary conditions) such as severe combined immunodeficiency (SCID), Omenn syndrome, and Cartilage-hair hypoplasia.

Causes of partial insufficiencies of T cell function include acquired immune deficiency syndrome (AIDS), and hereditary conditions such as DiGeorge syndrome (DGS), chromosomal breakage syndromes (CBSs), and B-cell and T-cell combined disorders such as ataxia telangiectasia (AT) and Wiskott-Aldrich syndrome (WAS).

Primary versus secondary

Primary (or hereditary) immunodeficiencies of T cells include some that cause complete insufficiency of T cells, such as severe combined immunodeficiency (SCID), Omenn syndrome, and Cartilage-hair hypoplasia.

Secondary (or acquired) causes are mainly:

• Marrow and other transplantation
• AIDS
• Cancer chemotherapy
• Lymphoma
• Glucocorticoid therapy

Overall, secondary causes are more common than primary ones.

Thymic versus peripheral

Causes of T cell deficiency by thymic hypoplasia can, in turn, be classified as:

• hypoparathyroid (such as Di George's syndrome)
• euparathyroid (Such as Nezelof syndrome and Ataxia telangiectasia)

Peripheral (or non-thymic) causes of T cell deficiency include: Purine nucleoside phosphorylase deficiency, hyper IgM syndrome (in turn including, for example, 1)

Effects

Pathogens of concern

Further information: Intracellular pathogen

The main pathogens of concern in T cell deficiencies are intracellular pathogens, including Herpes simplex virus, Mycobacterium and Listeria. Also, intracellular fungal infections are also more common and severe in T cell deficiencies.

Other intracellular pathogens of major concern in T cell deficiency are:

• Mycobacterium avium intracellulare
• Salmonella species
• Rhodococcus equi
• Pneumocystis jirovecii
• Toxoplasma gondii
• Cryptosporidium parvum
• Leishmania species
• Herpesviridae, mainly herpes simplex, cytomegalovirus and varicella zoster
• JC virus
• Cryptococcus neoformans
• Histoplasma capsulatum
• Systemic yeast infections

Presentations

Presentations differ among causes, but T cell insufficiency generally manifests as unusually severe common viral infections (e.g. by respiratory syncytial virus (RSV), enterovirus, rotavirus), mucocutaneous candidiasis, diarrhea, and eczematous or erythrodermatous rashes. Failure to thrive and cachexia are late signs of a T-cell defect.