An IEI gene may be defective because it is not expressed normally. These expressions may be under expressed, over-expressed, or a gene can be formed with reduced, increased, or without any activity [8, 17]. The defective IEI gene in parents may not be expressed in their offspring due its location in the X or Y chromosome, or one of 46 remaining non-sex chromosomes (known as autosomes) [8, 17]. The offspring who inherit IEI gene may not show symptoms due to the following reasons [17, 18].

The gene is under expressed (reduced penetrance) or not expressed (non-penetrance) in males or females.

A committee of the IEI was established by the World Health Organization (WHO) in 1973 [19]. The objectives of this committee were to describe and classify the types of primary immune defects or diseases affecting humans [19]. In the 1990s, WHO decided to focus on more common diseases of IEI, and a committee was formed known as the International Union of Immunological Societies (IUIS). In the 1980s [20], the number of genes when mutated to cause specific IEI disorders was < 10 but by 2022 the number rose to 485 mutated genes causing these disorders [21]. It is anticipated that with more DNA sequencing the number of mutated genes will increase. In 2023, the number of IEIs was estimated to be between 1 in 1,000 and 1 in 5,000 individuals but this may be as high as 1 in 500 individuals [3, 22].

The IUIS report was updated in 2022 (previously published January 2020) and in this report the classification of IEI consisted of key clinical and laboratory features of 55 monogenic gene defects and 1 phenocopy due to autoantibodies [2]. The report indicated that in 2022 there were 499 (increased from 485) known genetic defects [21]. It is expected that this report will help immunologists and geneticists to pursue and identify cellular and molecular mechanisms of monogenic infections and related human immune disorders [2]. Currently, IUIS (2022 report) classified IEI into 10 categories and these are described below [2, 21, 22]. These categories are summarized in Table 1.

Combined immunodeficiencies (cellular and hormonal immunodeficiencies) consisting of 66 defective genes and 58 diseases. The diseases in this classification are SCID diseases that are associated with low levels of CD3 protein-expressing T cells.

Impaired antibody production can lead to X-linked agammaglobulinemia, common variable immunodeficiency, X-linked or autosomal hyper IgM syndrome, and selective IgA deficiency [22]. Complications of PIDs can lead to autoimmunity and malignancies and after infection cancer is the second most leading cause of death in patients with PIDs [2, 6]. Early diagnosis and treatment of IEI is very important [23]. IEI can be divided into two groups:

Immune deficiency disorders (infections): For example, current or recurrent infections, severe infections, unusual pathogens, and restricted pathogen pattern [24].

Immunodeficiency can be classified as [25]:

Primary immunodeficiency: Genetically determined and typically occur during infancy or childhood. Primary immunodeficiency can be characterized by humoral or cellular immunity, combined humoral and cellular immunity, phagocytic cells, or complement proteins.

Initial tests to diagnose immune disorders should include [25]:

Complete blood count (CBC).

The standard humoral immune function test is the measurement of Ig levels in blood [26]. There are five classes of Igs in humans. These are known as IgG, IgM, IgA, IgE, and IgD, IgG being the most abundant of these Igs [27]. The tests include IgG, IgA, IgM, and sometimes IgE levels. The results should be compared with age- and sex-matched controls [26].

There are two tests which are conducted to evaluate the specific antibody production [26].

The T cell dependent pathway measured by the antibody response to protein antigens, such as tetanus and diphtheria toxoids.

There is an additional test to evaluate the antibody deficiencies [26]. This test evaluates different types of lymphocytes in the blood using flow cytometry. B cells produce antibodies and its absence can be attributed to disorders of antibodies such as X-linked agammaglobulinemia [26]. In addition, DNA testing can be done to check for a genetic defect for the child to be born so that treatment of the defect can begin immediately [25, 26]. Genetic testing can confirm a variant in the gene encoding Bruton tyrosine kinase (BTK) associated with X-linked agammaglobulinemia [26]. Furthermore, Ig production can be assessed by cultured lymphocytes in response to a variety of different kinds of stimuli [26].

Immune Deficiency Foundation in its laboratory tests states [26] “It is important to note that in a person with a previously confirmed defect in antibody production, stopping therapy to recheck for antibody levels and immunization response is unnecessary and may place the individual at risk of acquiring an infection during the period when the Ig replacement therapy is stopped. In someone whose diagnosis of an antibody immunodeficiency is unclear, it may be necessary to stop Ig replacement therapy for a period of four to six months so that the individual’s humoral immunity can be adequately assessed”.

Cellular or T cell immunity determines the number of different types of T cells and evaluate the function of these cells. In the USA, there is newborn screening test which screen the levels of T cells [26]. The infants may be healthy looking but this screening test helps in detecting treatable genetic defects [26]. This newborn screening test makes it easier to treat SCID and other related severe T cell immunodeficiencies since infants with these conditions were identified at birth [26].

CBC test is the simplest method to evaluate the decrease or absence of T cells. The total blood lymphocyte count is an appropriate test for the assessment of decreased T cell numbers. The rationale behind this is that since 75% of the circulating lymphocytes are T cells and a reduction in T cells indicates that the total number of lymphocytes or total lymphocyte count has decreased [26].

Neutrophil function test focuses on the white blood cells counts with differentials. This test determines if there is neutropenia that is a decrease in the absolute neutrophil count [26]. If these initial screening tests of neutrophil numbers are found normal then the testing is focused on two PIDs. Chronic granulomatous disease (CGD) and leukocyte adhesion deficiency (LAD). Both of these disorders have normal or elevated numbers of neutrophils. At this time, for testing both of these two disorders, flow cytometry is considered reliable [26].

The standard screening test for complement deficiency is the total hemolytic complement assay or CH50 [26]. The CH50 will be almost negative if there is a defect in one complement component. In rare cases, there may be defects in other complement pathway that can be screened by a functional test directed specifically at this pathway, the AH50 test [26].

Innate immunity tests determine the number and activity of lymphocytes such as natural killer (NK) cells and the function of various cell surface receptors [26].

Laboratory immunological testing initially confirms IEIs by showing altered numbers or function of T cells, B cells, neutrophils, complement or innate immune system [28–37]. Since 1985, the genetic defects leading to IEIs led to the molecular confirmation of many immunological diagnoses of IEIs. These days the evaluation of IEIs mainly relies on direct genetic confirmation, and genetic testing has become an integral part of diagnostic process of IEIs [9, 17]. Genetic tests help in rapid screening for mutations in hundreds of genes that affect the immune system [26].

It appears that there is too much focus on the infectious diseases in IEI but one should also focus on other diseases related to IEI [38]. These diseases can be manifestations of early onset and/or refractory to conventional treatments [39], severe and difficult to control allergic conditions [40], recurrent or persistent inflammatory processes with or without fever [41], and malignant diseases [42]. Considering these onset of diseases, early diagnosis is of immense importance to reduce morbidity and mortality from these diseases [43]. It has been also recommended that screening for SCID using T cell receptor excision circle (TREC) and kappa recombining excision circles (KREC) be incorporated in neonatal screening programs [44].

The aforementioned tests are initial evaluation of IEI but with the discovery of many other diseases associated with IEI new tests may be needed for other defects [45]. Some of these are summarized below.

The measurement of the enzymatic activity of adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP) in peripheral blood can help in the diagnosis of SCID. Furthermore, the measurement of uric acid can identify PNP deficiency (very low levels of uric acid) [46].

IEM diseases are inherited biochemical disorders with specific enzyme defects that interfere with normal metabolism of a person. IEM diseases are classified as disorders of carbohydrate metabolism, protein metabolism, organic acid metabolism, fatty acid oxidation, glycogen, or lysosomal storage diseases [54, 55]. The major subtypes of IEM can be classified into three broad groups according to the pathogenesis and clinical presentations. These classifications are as follows.

Antimicrobial prophylaxis is regularly used in the treatment for CGD or leucocyte adhesion deficiency [71]. These patients have multiple deep skin infections. Antimicrobial prophylaxis treatment is based on the known risk of infection. Infants with severe SCID are typically given multiple forms of antimicrobial prophylaxis such as trimethoprim/sulfamethoxazole [71]. It should be however, recognized the adverse effects (mainly skin rashes, allergy, photosensitivity, and kidney damage) of sulfonamides in infants, children, and elderly. Infants with SCID due to their immunocompromised status are also given prophylaxis against viral and fungal infections such as acyclovir and fluconazole, respectively. However, patients with antibody deficiencies are treated at higher doses or longer period of antimicrobial dosing than those patients who do not have antibody deficiencies [71].

Practice parameter for the Diagnosis and Management of PIDs states: “The standard dose and duration of antimicrobial regimens might not be adequate to eradicate infections in immunocompromised hosts. Early combined antimicrobial therapy and prolonged courses should be considered.” [72]. The selection of an antimicrobial agent based on the culture of an adequate sample (sputum or tissue sample) is considered the preferred approach [71].

In children with antibody deficiency, doses of 20 mg/kg once or twice daily, trimethoprim/sulfamethoxazole given either daily or three times weekly at 5 mg/kg, or azithromycin 5 mg/kg thrice weekly has been found to be beneficial. In adults with antibody deficiency, either amoxicillin 500–1,000 mg daily or twice daily, trimethoprim/sulfamethoxazole 160 mg daily or twice daily, or azithromycin 500 mg once weekly or 250 mg every other day were found to be beneficial [72, 73].

Although, prophylactic antibiotics in primary antibody deficiency is widely used, there are very little data to show the effectiveness of these antibiotics. There are no controlled studies on the use of adjunct prophylactic antibiotics in patients with PID [73].

IEIs can lead to increased susceptibility to fungal infections. Patients with CGD deficiency can suffer from aspergillosis [74]. Mold-active antifungal prophylaxis should be prescribed to these patients immediately after diagnosis [74]. Most patients with IEI and invasive fungal infections can be managed using prolonged duration of antimicrobial prophylaxis and antifungals [74].

Following Bruton’s [16] use of Ig to treat an 8-year-old boy suffering from pneumonia and other bacterial infections, Igs have become an integral part of PID treatment. Today, Ig products are widely used for the treatment of immunodeficiency syndromes and autoimmune diseases such as chronic inflammatory demyelinating polyneuropathy (CIDP), immune thrombocytopenic purpura (ITP), and PID [1, 75]. Almost 75% of PIDs (impaired antibody production) are treated with Igs. The objective of the Ig replacement therapy is to maintain stable and optimum concentrations of IgGs for the therapeutic or clinical management of patients with PID [76].

Igs are endogenous proteins produced by B lymphocyte cells. In humans, there are five classes of Igs: IgG, IgM, IgA, IgE, and IgD [3, 4]. The IgG is the most abundant of these Igs and are further divided into four subclasses: IgG1, IgG2, IgG3, and IgG4 [77, 78]. Igs are manufactured from pooled human plasma and contain more than 95% IgG with intact Fc-dependent effector functions [77, 78].

Oral administration of IgGs is not possible because IgGs are unstable in the acidic pH of the stomach and also due to the proteolytic degradation in the stomach and intestine [79]. In early days, IgGs were administered by intramuscular route but these days intramuscular administration is rarely used. These days, IgGs are administered either by intravenous (IV) or subcutaneous (SC) route [79].

IgGs are distributed to the kidneys, lungs, liver, spleen, heart, and lymph. The distribution of IgGs to the tissues takes place by the lymph [80, 81]. Since IgGs are intact antibodies and are not filtered by the kidneys therefore, their elimination occurs via catabolism. Other elimination pathways are binding to specific receptors, receptor-mediated endocytosis, recycling via FcRn, non-specific uptake by tissues, binding via Fc receptors, and anti-product antibodies [80, 81]. Antibody fragments are filtered and reabsorbed as well as metabolized by proximal tubular cells. Receptor-mediated drug disposition generally leads to the saturation of IgG clearance resulting in nonlinear pharmacokinetics (PK) of IgGs [80, 81].

The IV route of administration is preferable over SC mainly because larger volumes of IgGs can be administered [79]. However, SC mode of administration is becoming popular due to the ease of administration. The FDA guidance [82] suggests a dose of IVIG ranging from 200 to 800 mg/kg every 3 or 4 weeks. The minimum trough level required for protection against bacterial infection is considered to be 500 mg/dL [82]. However, studies have shown that IgG concentrations of 700 to 1,000 mg/dL, are more efficient to prevent infections, particularly pneumonia [76, 83, 84]. Higher IgG concentrations provide protection from infection especially, to patients with chronic pulmonary disease [85–87]. IgG concentrations widely vary among patients therefore, treatment should be individualized. It is very important that IgG dose, trough level, and the risk of serious and non-serious bacterial infections be established in order to provide right dose to the patients with PID.

Although, the SC route of administration is becoming popular but many caveats of SC administration of IgGs should be recognized [79, 88]. The dosing frequency of SCIg is weekly and the increase in IgG concentrations is not as rapid as IVIg. It takes several days to reach the maximum concentrations of IgGs. Large doses of IgGs (> 500 mg/dL) cannot be administered due to the limited solubility of IgG products [79, 88]. Another issue with the SC administration is the adjustment of bioavailability of SCIgs [82]. The concentrations of SCIgs are lower than the concentrations of IVIG [89]. This leads to lower trough and area under the curve (AUC) values of SCIg products than the IVIg products. The US FDA [82] recommends that SCIg dose be adjusted when patients are switched from IVIg to SCIg based on the patients previous IVIg dose. The average dose adjustment for SCIg is generally from 130% to 153% depending on the product [90]. On the other hand, the European studies do not account for the SCIg dose adjustment based on the bioavailability of SC product [91].

PK plays a very important role in modern day drug development irrespective of small or large molecules [92]. PK-based dosing of Igs is now a regular clinical practice and is advocated in the literature [82, 91–94]. The PK of Igs is characterized from plasma concentration-time profiles and from these profiles several PK parameters are derived. These parameters can be half-life, area under the curve, clearance, volume of distribution, maximum and (C_max) and minimum (C_min) concentrations [90, 93]. The PK of IgGs has been extensively studied in adults following IVIg and SCIg administration [79, 91–94]. Although, Igs are also used in neonates, infants, and children but limited PK data are available for these age groups [95–98].

Several factors such as age (neonates, infants, and children), obesity, pregnancy, and immunogenicity can influence the PK and dosing of Igs. Therefore, these factors should be taken into account [90, 93, 94, 97]. In Table 2, the half-life and clearance values of some IgGs are shown.

In SCID, one of the treatment options is allogeneic HSCT [99]. In a patient with IEI, the patient’s hematopoietic stem cells (HSCs) are replaced by healthy donor HSCs [99]. Since the first successful HSCT in 1968 in patients with SCID, HSCT has emerged as a lifesaving therapy [99]. However, HSCT in many cases has resulted as a partial or insufficient cure mainly due to graft failure (GF) [100]. The risk of GF is known to be higher in non-malignant disorders and 8–16% patients with IEI were reported to suffer from GF in multi-center studies [101, 102]. A second allogenic HSCT may be an option for GF. Laberko et al. [100] conducted a study in 48 patients with IEI who received a second HSCT and the authors found the results encouraging. The authors’ conclusion was that an individualized approach is required for the second HSCT in patients with IEI [100].

The objective of gene therapy (GT) is to fix, correct, or modify a faulty gene and replace it with a healthy gene in order to cure diseases or make the body better able to fight diseases [99]. GT offers an alternative to HSCT by modifying autologous hematopoietic stem cells. This method does not require a suitable allogeneic donor and eliminates the risk of graft versus host disease [99]. There are two approaches in GT: ex-vivo or in-vivo GT therapy. In ex-vivo GT therapy, generally, CD34⁺ hematopoietic stem and progenitor cells are genetically modified outside the body and then transplanted the cells back into the patients. For in-vivo gene therapy, the genetic material is directly delivered to target cells in the body [103]. Viral gene addition (viral vectors) currently in use for IEIs or gene editing are two main approaches for gene therapy for the treatment or management of IEIs [99].

Phenotype is described as an individual’s observable trait(s) or character(s). A person’s phenotype such as height, color of eyes and hair, and blood type is determined by both their genomic makeup (genotype) and environmental factors [107].

Reverse genetics is used to understand the function(s) of a gene by examining the changes to phenotypes of cells or organisms. The alteration in phenotypes is caused by genetically engineering specific nucleic acid sequences within the gene (DNA or RNA). Mutations are induced by radiations or chemicals and mutant cells or organisms are isolated based on their phenotype and genomic mapping is performed to confirm phenotype to genetics [107, 108].

The reverse genetics is opposite to forward genetics. In forward genetics, one attempts to find the genetic basis of a phenotype or trait, whereas, in reverse genetics one attempts to find the phenotypes that are influenced by particular genetic sequences [107, 108].

Reverse genetics is a powerful tool that can be used to understand the function of genes so that new treatments for genetic disorders and diseases can be developed. Reverse genetics has its therapeutic benefits. The main therapeutic application of reverse genetics is in the engineering of vaccines against viruses and in the reconstruction of a viral genome [108]. Live attenuated vaccines have higher immunogenicity than traditional inactivated vaccines and reverse genetics can be used to modify viral genomes by altering their pathogenicity in the development of live attenuated vaccines [108]. Besides, it’s application in vaccine development, the therapeutic applications of reverse genetics are in GT, gene editing, cancer therapy, and protein engineering.

Gene reversal, also known as gene conversion is a process in which an inactivated gene is reactivated [109]. For example, effects of a genetic mutation or disease can be reversed by inserting a normal copy of the gene into cells or organisms. However, the drawback of a gene reversal is that the process can lead to loss or alteration of genetic information affecting an organism’s development and function. It should be noted that the gene reversal and reverse genetics are two different things.

Gene reversal is in the early stages of research and it will take some time to understand its full therapeutic benefit. At the moment, gene reversal technique is focused on gene therapy, cancer treatment, regenerative medicine, treatment of congenital defects, aging, and management or cure of genetic disorders [109].