Epstein-Barr (EBV) virus
A very common virus that nearly all of us will meet at some stage or other. In people with normal immune systems, the virus causes glandular fever; usually a mild illness causing fever, tiredness and sore throat. The virus specifically attacks certain white cells called B lymphocytes causing them to multiply in an uncontrolled manner. The healthy immune system fights the virus but the virus remains dormant in the body and may emerge again during times of illness or old age.

SAP
This is the protein missing in XLP, in most families, the mistake causing this protein not to be produced is in a gene called SH2D1A.. SAP is found within cells and carries messages from the surface of the cell to other proteins inside the cell. The function of the gene is not exactly known but it is thought to control the way certain immune cells respond to and fight infection. If SAP is defective, the immune system does not fight infection properly leading to severe problems.

T cells
Very important cells of the immune system that fight infection especially viral infections. T cells are important also in helping other immune cells such as B cells to make antibodies to protect against infection. If T cells are absent or do not work properly, then individuals are very vulnerable to infective problems.

NK cells
Another important immune cell which has a major role in fighting viral infection and also protects us against developing certain types of tumours.

Gene therapy
The introduction of new genes into cells or tissues to try and correct disease. Gene therapy has recently shown great success in correcting certain forms of immunodeficiency.

Gene therapy vector
A gene therapy vector carries new genes into cells. Often these vectors are disabled viruses which have been modified to carry the gene of interest. These viral vectors can get inside the cell and place the new gene into the chromosome of the cell thereby modifying the cell with new genetic material.