Conditions that cause abnormal or absent T cells will result in immunodeficiency. Examples of these disorders are discussed below.
DiGeorge syndrome, also called 22q11.2 deletion syndrome, results from a microdeletion in a portion of chromosome 22 that leads to a developmental defect in the third pharyngeal pouch and third branchial cleft. One of the consequences of this is incomplete development of the thymus gland. An underdeveloped thymus gland results in an inability to create functional T cells.
Features of DiGeorge syndrome can be remembered with the CATCH-22 mnemonic:
- C – Congenital heart disease
- A – Abnormal facies (characteristic facial appearance)
- T – Thymus gland incompletely developed
- C – Cleft palate
- H – Hypoparathyroidism and resulting Hypocalcaemia
- 22nd chromosome affected
Purine Nucleoside Phosphorylase Deficiency
Purine nucleoside phosphorylase (PNP) deficiency is an autosomal recessive condition. PNPase is an enzyme that helps breakdown purines. Without this enzyme, a metabolite called dGTP builds up. This metabolite is exclusively toxic to T cells. Increased levels of dGTP causes low levels of T-lymphocytes. There are normal levels of B cells and immunoglobulins. Clinically, patients immunity to infection gradually gets worse. They become increasingly susceptible to infections, particularly viruses and live vaccines.
Wiskott-Aldrich syndrome (WAS) is an X-linked recessive condition with a mutation on the WAS gene. It causes abnormal functioning of T cells. Other features include:
- Recurrent infections
- Chronic bloody diarrhoea
Ataxic telangiectasia is an autosomal recessive condition affecting the gene coding for the ATM serine/threonine kinase protein on chromosome 11. This protein is important in several functions of DNA coding, meaning that a mutation in this gene leads to problems coding for many other genes.
There are various features of the condition:
- Low numbers of T-cells and immunoglobulins, causing immunodeficiency and recurrent infections.
- Ataxia: problems with coordination due to cerebellar impairment
- Telangiectasia, particularly in the sclera and damaged areas of skin
- Predisposition to cancers, particularly haematological cancers
- Slow growth and delayed puberty
- Accelerated ageing
- Liver failure
Acquired Immunodeficiency Syndrome
AIDS is caused by infection with HIV, which reduces the numbers of CD4 T-cells. This is covered in detail in the infectious disease section.
Last updated January 2020