Activated PI3K-delta syndrome (also known as APDS) 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. The severity of activated PI3K-delta syndrome varies widely. Some people may have multiple, severe infections while others show mild symptoms to none at all.\n\nThere are two types of activated PI3K-delta syndrome, each with different genetic causes.\n\nMost commonly, people with activated PI3K-delta syndrome develop recurrent infections that begin in childhood, 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, such as Epstein-Barr virus, herpes simplex virus, or cytomegalovirus infections.\n\nAnother possible feature of activated PI3K-delta syndrome is abnormal clumping of white blood cells. These clumps can lead to enlarged lymph nodes (lymphadenopathy) or an enlarged spleen (splenomegaly). The white blood cells can also build up to form solid masses (nodular lymphoid hyperplasia), usually in the moist lining of the airways or intestines. While nodular lymphoid hyperplasia is not cancerous (benign), activated PI3K-delta syndrome increases the risk of developing forms of blood cancer called Hodgkin lymphoma and non-Hodgkin lymphoma.\n\nSome people with activated PI3K-delta syndrome develop autoimmunity, which occurs when the body attacks its own tissues and organs by mistake.

Immunodeficiency-36 with lymphoproliferation (IMD36) is an autosomal dominant primary immunodeficiency with a highly heterogeneous clinical phenotype, characterized primarily by recurrent respiratory tract infections, lymphoproliferation, and antibody deficiency. Other features include growth retardation, mild neurodevelopmental delay, and autoimmunity. The major complication is development of B-cell lymphoma (Elkaim et al., 2016).

Immunodeficiency-62 (IMD62) is an autosomal recessive primary immunologic disorder clinically characterized by onset of recurrent upper and lower respiratory infections late in the first decade of life. Patients may also have increased viral susceptibility to varicella zoster virus (VZV) or herpes simplex virus (HSV). Laboratory studies show impaired antibody response to vaccination, low levels of circulating memory B cells, and almost undetectable antibodies. There is also evidence of secondary T-cell dysfunction. The disorder may result from disturbed actin cytoskeleton dynamics causing impaired lymphocyte migration (summary by Bouafia et al., 2019).