Activated PI3K-delta syndrome is a disorder that impairs the immune system. Individuals with this condition often have low numbers of white blood cells (lymphopenia), particularly B cells and T cells. Normally, these cells recognize and attack foreign invaders, such as viruses and bacteria, to prevent infection. Beginning in childhood, people with activated PI3K-delta syndrome develop recurrent infections, particularly in the lungs, sinuses, and ears. Over time, recurrent respiratory tract infections can lead to a condition called bronchiectasis, which damages the passages leading from the windpipe to the lungs (bronchi) and can cause breathing problems. People with activated PI3K-delta syndrome may also have chronic active viral infections, commonly Epstein-Barr virus or cytomegalovirus infections.
Activated PI3K-delta syndrome is caused by mutations in the PIK3CD gene, which provides instructions for making a protein called p110 delta (p110δ). This protein is one piece (subunit) of an enzyme called phosphatidylinositol 3-kinase (PI3K), which turns on signaling pathways within cells. The version of PI3K containing the p110δ subunit, called PI3K-delta, is specifically found in white blood cells, including B cells and T cells.
