Inborn errors of immunity

Inborn errors of immunity (IEI) are a heterogenous group of disorders in which a mutation in any one of various genes that regulate the immune system causes increases in the susceptibility of individuals to develop a dysfunction in their immune system.^[1][2] (As used here, mutations include deletions or other changes in any part of a gene that causes it to be dysfunctional.) Depending on the gene involved, this dysfunction may induce the development of an: a) autoinflammatory disease by causing a malfunction in the innate immune system; b) autoimmune disease by causing a malfunction in the adaptive immune system;^[3] c) viral, bacterial, fungal, or mycobacterial infection by causing a malfunction in one of the various components of the immune system that combat these pathogens;^[3][4] d) allergic disease by causing a hypersensitive immune system that overreacts to otherwise harmless substances; e) lose of one or more types of circulating blood cells by causing a failure of the bone marrow to produce the circulating blood cell type(s); f) hematological cancers by causing a mutation in any one of various oncogenes (i.e., genes with the potential to cause a cancer); g) non-hematological cancers as well as hematological cancers by causing a mutation in the ATM serine/threonine kinase gene (these cancers are mainly pancreatic cancer, prostate cancer, stomach cancer and invasive ductal carcinoma of the breast;^[5] see cancers in ATM serine/threonine kinase gene defects);^[3][6][7] and h) non-malignant lymphoproliferative disorders by causing the excessive proliferation of T-cell or B-cell lymphocytes in the lymph nodes, gastrointestinal tract, liver, skin, or more than one of these organs.^[8]

A human immune disease that would later be classified as an IEI was first defined by Ogden Bruton. In the early 1950s, he examined an 8-year-old boy who had 19 episodes of pneumonia over a period of 4 years. Expecting that individuals with such a history of repeated infections would have high levels of infection-fighting antibodies in their serum, Dr. Bruton was surprised to find that the boy had hypogammaglobulinemia, i.e., his serum lacked detectible levels of circulating antibodies which attack infection-causing microorganisms and virus.^[9][10] That same year, Dr. Bruton and colleagues published on two other infection-prone patients who also lacked detectable levels of these serum antibodies^[10][11] This particular from of hypogammaglobulinemia, now termed X-linked agammaglobulinemia and characterized as an IEI, occurs in about 1 per 379,000 live births.^[12][13] It is also termed Bruton's agammaglobulinemia and the gene that when mutated causes this disease is termed the Bruton's tyrosine kinase, i.e., BKT, gene. The product of this gene, the BTK protein, contributes indirectly to promoting the production of all the antibody subtypes, i.e., IgG, IgA, IgM, and IgE.^[14]

Impairments in the immune system's protective actions have been referred to as primary immunodeficiencies (PID), i.e., immune deficiencies that are present at birth and not caused by secondary factors such as other diseases or exposure to genotoxic agents.^[15] The PID disorders (see List of primary immunodeficiencies) and its subgroup, the primary immune regulatory disorders (PIRDs; i.e., disorders of immunity characterized as excessive proliferations of lymphocytes and the development of immune responses against one's own normal tissues^[16]), are immune disorders similar to those in IEI.^[2][17] Finally, inborn errors of metabolism (i.e., IEM) are a group of about 1700 disorders caused by a mutation in any one of about 1500 genes that causes a defect in a pathway that metabolizes proteins, fats, or carbohydrates or that impairs the function of a subcellular organelle. This mutation usually causes a complicated medical condition involving several human organ systems.^[18][19] When any one of the disorders in the PID, PIRDs, or IEM classifications is caused by a single gene mutation that disrupts the immune system, it is termed an IEI. Consequently, many IEIs are also termed a PID, PIRDs, and/or IEM.^{[2][17][18][19]}

In 1973, the World Health Organization (WHO) established the Inborn Errors of Immunity Committee for the purpose of classifying and identifying immune defects in humans. The committee focused on rare immune diseases. In the 1990s, the WHO decided to focus on more common diseases, and the committee was taken on by the International Union of Immunological Societies (i.e., IUIS). This relationship was made official in 2008.^[20] The number of genes that when mutated to cause specific IEI disorders has steadily rose from less than 10 in the 1980s^[21] to the IUIS expert committee's 2022 classification of 485 mutated genes causing these disorders.^[22] These numbers are expected to increase further as DNA sequencing using automated methods (e.g., massive parallel sequencing), further studies of less severe immune disorders, and analyses of multiple tissues in individuals that may have carry the dysfunctional gene in some but not their tissues (see mosaicism). Thus, the prevalence of IEIs in 2023 was estimated to be between 1 in 1,000 and 1 in 5,000 individuals but this may be an underestimate: its true prevalence may turn out to be as high as 1 in 500 individuals.^[2][23][24]