Primary immunodeficiency

Primary immunodeficiencies are disorders in which part of the body's immune system is missing or does not function properly. To be considered a primary immunodeficiency, the cause of the immune deficiency must not be secondary in nature (i.e., caused by other disease, drug treatment, or environmental exposure to toxins). Most primary immunodeficiencies are genetic disorders; the majority are diagnosed in children under the age of one, although milder forms may not be recognized until adulthood. About 1 in 500 people is born with a primary immunodeficiency.

Signs and symptoms
The precise symptoms of a primary immunodeficiency depend on the type of defect. Generally, the symptoms and signs that lead to the diagnosis of an immunodeficiency include recurrent or persistent infections or developmental delay as a result of infection. Particular organ problems (e.g. diseases involving the skin, heart, facial development and skeletal system) may be present in certain conditions. Others predispose to autoimmune disease, where the immune system attacks the body's own tissues, or tumours (sometimes specific forms of cancer, such as lymphoma). The nature of the infections, as well as the additional features, may provide clues as to the exact nature of the immune defect.

Diagnosis
The basic tests performed when an immunodeficiency is suspected should include a full blood count (including accurate lymphocyte and granulocyte counts) and immunoglobulin levels (the three most important types of antibodies: IgG, IgA and IgM).

Other tests are performed depending on the suspected disorder:
 * Quantification of the different types of mononuclear cells in the blood (i.e. lymphocytes and monocytes): different groups of T lymphocytes (dependent on their cell surface markers, e.g. CD4+, CD8+, CD3+, TCRαβ and TCRγδ), groups of B lymphocytes (CD19, CD20, CD21 and Immunoglobulin), natural killer cells and monocytes (CD15+), as well as activation markers (HLA-DR, CD25, CD80 (B cells).
 * Tests for T cell function: skin tests for delayed-type hypersensitivity, cell responses to mitogens and allogeneic cells, cytokine production by cells
 * Tests for B cell function: antibodies to routine immunisations and commonly acquired infections, quantification of IgG subclasses
 * Tests for phagocyte function: reduction of nitro blue tetrazolium chloride, assays of chemotaxis, bactericidal activity.

Due to the rarity of many primary immunodeficiencies, many of the above tests are highly specialised and tend to be performed in research laboratories.

Criteria for diagnosis were agreed in 1999. For instance, an antibody deficiency can be diagnosed in the presence of low immunoglobulins, recurrent infections and failure of the development of antibodies on exposure to antigens. The 1999 criteria also distinguish between "definitive", "probable" and "possible" in the diagnosis of primary immunodeficiency. "Definitive" diagnosis is made when it is likely that in 20 years, the patient has a >98% chance of the same diagnosis being made; this level of diagnosis is achievable with the detection of a genetic mutation or very specific circumstantial abnormalities. "Probable" diagnosis is made when no genetic diagnosis can be made, but the patient has all other characteristics of a particular disease; the chance of the same diagnosis being made 20 years later is estimated to be 85-97%. Finally, a "possible" diagnosis is made when the patient has only some of the characteristics of a disease are present, but not all.

Conditions
The International Union of Immunological Societies recognises eight classes of primary immunodeficiencies, totaling over 120 conditions. Their most recent update retained the classification into eight groups, while adding several new conditions into these groups (such as coronin-1A deficiency, immunodeficiency with centromeric instability and facial anomalies, and defects of Ficolin 3).

Table I: Combined T and B–cell immunodeﬁciencies
In these disorders both T lymphocytes and often B lymphocytes, regulators of adaptive immunity, are dysfunctional or decreased in number. The main members are various types of severe combined immunodeficiency (SCID).
 * 1) T-/B+ SCID (T cells predominantly absent): γc deficiency, JAK3 deficiency, interleukin 7 receptor chain α deficiency, CD45 deficiency, CD3δ/CD3ε deficiency.
 * 2) T-/B- SCID (both T and B cells absent): RAG 1/2 deficiency, DCLRE1C deficiency, adenosine deaminase (ADA) deficiency, reticular dysgenesis
 * 3) Omenn syndrome
 * 4) DNA ligase type IV deficiency
 * 5) Cernunnos deﬁciency
 * 6) CD40 ligand deficiency
 * 7) CD40 deficiency
 * 8) Purine nucleoside phosphorylase (PNP) deficiency
 * 9) CD3γ deficiency
 * 10) CD8 deficiency
 * 11) ZAP-70 deficiency
 * 12) Ca++ channel deﬁciency
 * 13) MHC class I deficiency
 * 14) MHC class II deficiency
 * 15) Winged helix deficiency
 * 16) CD25 deﬁciency
 * 17) STAT5b deﬁciency
 * 18) Itk deﬁciency
 * 19) DOCK8 deﬁciency

Table II: Predominantly antibody deﬁciencies
In antibody deficiencies, one or more isotypes of immunoglobulin are decreased or don't function properly. These proteins, generated by plasma cells, normally bind to pathogens, targeting them for destruction.
 * 1) Absent B cells with a resultant severe reduction of all types of antibody: X-linked agammaglobulinemia (btk deficiency, or Bruton's agammaglobulinemia), μ-Heavy chain deficiency, l 5 deficiency, Igα deficiency, BLNK deficiency, thymoma with immunodeficiency
 * 2) B cells low but present or normal, but with reduction in 2 or more isotypes (usually IgG & IgA, sometimes IgM): common variable immunodeficiency (CVID), ICOS deficiency, CD19 deficiency, TACI (TNFRSF13B) deficiency, BAFF receptor deficiency.
 * 3) Normal numbers of B cells with decreased IgG and IgA and increased IgM: Hyper-IgM syndromes
 * 4) Normal numbers of B cells with isotype or light chain deficiencies: heavy chain deletions, kappa chain deficiency, isolated IgG subclass deficiency, IgA with IgG subsclass deficiency, selective immunoglobulin A deficiency
 * 5) Specific antibody deficiency to specific antigens with normal B cell and normal Ig concentrations
 * 6) Transient hypogammaglobulinemia of infancy (THI)

Table III: Other well deﬁned immunodeﬁciency syndrome
A number of syndromes escape formal classification but are otherwise recognisable by particular clinical or immunological features.
 * 1) Wiskott-Aldrich syndrome
 * 2) DNA repair defects not causing isolated SCID: ataxia telangiectasia, ataxia-like syndrome, Nijmegen breakage syndrome, Bloom syndrome
 * 3) DiGeorge syndrome (when associated with thymic defects)
 * 4) Various immuno-osseous dysplasias (abnormal development of the skeleton with immune problems): cartilage-hair hypoplasia, Schimke syndrome
 * 5) Hermansky-Pudlak syndrome type 2
 * 6) Hyper-IgE syndrome
 * 7) Chronic mucocutaneous candidiasis
 * 8) Hepatic venoocclusive disease with immunodeﬁciency (VODI)
 * 9) XL-dyskeratosis congenita (Hoyeraal-Hreidarsson syndrome)

Table IV: Diseases of immune dysregulaton
In certain conditions, the regulation rather than the intrinsic activity of parts of the immune system is the predominant problem.
 * 1) Immunodeficiency with hypopigmentation or albinism: Chediak-Higashi syndrome, Griscelli syndrome type 2
 * 2) Familial hemophagocytic lymphohistiocytosis: perforin deficiency, MUNC13D deficiency, syntaxin 11 deficiency
 * 3) X-linked lymphoproliferative syndrome
 * 4) Syndromes with autoimmunity:
 * 5) (a) Autoimmune lymphoproliferative syndrome: type 1a (CD95 defects), type 1b (Fas ligand defects), type 2a (CASP10 defects), type 2b (CASP8 defects)
 * 6) (b) APECED (autoimmune polyendocrinopathy with candidiasis and ectodermal dystrophy)
 * 7) (c) IPEX (immunodysregulation polyendocrinopathy enteropathy X-linked syndrome)
 * 8) (d) CD25 deﬁciency

Table V: Congenital defects of phagocyte number, function, or both
Phagocytes are the cells that engulf and ingest pathogens (phagocytosis), and destroy them with chemicals. Monocytes/macrophages as well as granulocytes are capable of this process. In certain conditions, either the number of phagocytes is reduced or their functional capacity is impaired.
 * 1) Severe congenital neutropenia: due to ELA2 deficiency (with myelodysplasia)
 * 2) Severe congenital neutropenia: due to GFI1 deficiency (with T/B lymphopenia)
 * 3) Kostmann syndrome
 * 4) Neutropenia with cardiac and urogenital malformations
 * 5) Glycogen storage disease type 1b
 * 6) Cyclic neutropenia
 * 7) X-linked neutropenia/myelodysplasia
 * 8) P14 deﬁciency
 * 9) Leukocyte adhesion deficiency type 1
 * 10) Leukocyte adhesion deficiency type 2
 * 11) Leukocyte adhesion deficiency type 3
 * 12) RAC2 deficiency (Neutrophil immunodeficiency syndrome)
 * 13) Beta-actin deficiency
 * 14) Localized juvenile periodontitis
 * 15) Papillon-Lefèvre syndrome
 * 16) Specific granule deficiency
 * 17) Shwachman-Diamond syndrome
 * 18) Chronic granulomatous disease: X-linked
 * 19) Chronic granulomatous disease: autosomal (CYBA)
 * 20) Chronic granulomatous disease: autosomal (NCF1)
 * 21) Chronic granulomatous disease: autosomal (NCF2)
 * 22) IL-12 and IL-23 β1 chain deficiency
 * 23) IL-12p40 deficiency
 * 24) Interferon γ receptor 1 deficiency
 * 25) Interferon γ receptor 2 deficiency
 * 26) STAT1 deficiency (2 forms)
 * 27) AD hyper-IgE
 * 28) AR hyper-IgE
 * 29) Pulmonary alveolar proteinosis

Table VI: Defects in innate immunity
Several rare conditions are due to defects in the innate immune system, which is a basic line of defence that is independent of the more advanced lymphocyte-related systems. Many of these conditions are associated with skin problems.
 * 1) Hypohidrotic ectodermal dysplasia
 * 2) NEMO deficiency
 * 3) IKBA deficiency
 * 4) EDA-ID
 * 5) IRAK-4 deficiency
 * 6) MyD88 deﬁciency
 * 7) WHIM syndrome (warts, hypogammaglobulinaemia, infections, myleokathexis)
 * 8) Epidermodysplasia verruciformis
 * 9) Herpes simplex encephalitis
 * 10) Chronic mucocutaneous candidiasis
 * 11) Trypanosomiasis

Table VII: Autoinﬂammatory disorder
Rather than predisposing for infections, most of the autoinflammatory disorders lead to excessive inflammation. Many manifest themselves as periodic fever syndromes. They may involve various organs directly, as well as predisposing for long-term damage (e.g. by leading to amyloid deposition).
 * 1) Familial Mediterranean fever
 * 2) TNF receptor associated periodic syndrome (TRAPS)
 * 3) Hyper-IgD syndrome (HIDS)
 * 4) CIAS1-related diseases:
 * 5) Muckle-Wells syndrome
 * 6) Familial cold autoinflammatory syndrome
 * 7) Neonatal onset multisystem inflammatory disease
 * 8) PAPA syndrome (pyogenic sterile arthritis, pyoderma gangrenosum, acne)
 * 9) Blau syndrome
 * 10) Chronic recurrent multifocal osteomyelitis and congenital dyserythropoietic anemia (Majeed syndrome)
 * 11) DIRA (deﬁciency of the IL-1 receptor antagonist)

Table VIII. Complement deﬁciencies
The complement system is part of the innate as well as the adaptive immune system; it is a group of circulating proteins that can bind pathogens and form a membrane attack complex. Complement deficiencies are the result of a lack of any of these proteins. They may predispose to infections but also to autoimmune conditions.
 * 1) C1q deficiency (lupus-like syndrome, rheumatoid disease, infections)
 * 2) C1r deficiency (idem)
 * 3) C1s deﬁciency
 * 4) C4 deficiency (idem)
 * 5) C2 deficiency (lupus-like syndrome, vasculitis, polymyositis, pyogenic infections)
 * 6) C3 deficiency (recurrent pyogenic infections)
 * 7) C5 deficiency (Neisserial infections, SLE)
 * 8) C6 deficiency (idem)
 * 9) C7 deficiency (idem, vasculitis)
 * 10) C8a deficiency
 * 11) C8b deficiency
 * 12) C9 deficiency (Neisserial infections)
 * 13) C1-inhibitor deficiency (hereditary angioedema)
 * 14) Factor I deficiency (pyogenic infections)
 * 15) Factor H deficiency (haemolytic-uraemic syndrome, membranoproliferative glomerulonephritis)
 * 16) Factor D deficiency (Neisserial infections)
 * 17) Properdin deficiency (Neisserial infections)
 * 18) MBP deficiency (pyogenic infections)
 * 19) MASP2 deficiency
 * 20) Complement receptor 3 (CR3) deﬁciency
 * 21) Membrane cofactor protein (CD46) deﬁciency
 * 22) Membrane attack complex inhibitor (CD59) deﬁciency
 * 23) Paroxysmal nocturnal hemoglobinuria
 * 24) Immunodeﬁciency associated with ﬁcolin 3 deﬁciency

Treatment
The treatment of primary immunodeficiencies depends foremost on the nature of the abnormality. This may range from immunoglobulin replacement therapy in antibody deficiencies—in the form of intravenous immunoglobulin (IVIG) or subcutaneous immunoglobulin (SCIG)—to hematopoietic stem cell transplantation for SCID and other severe immunodeficiences. Reduction of exposure to pathogens may be recommended, and in many situations prophylactic antibiotics may be advised.

Epidemiology
A survey of 10,000 American households revealed that the prevalence of diagnosed primary immunodeficiency approaches 1 in 1200. This figure does not take into account people with mild immune system defects who have not received a formal diagnosis.

Milder forms of primary immunodeficiency, such as selective immunoglobulin A deficiency, are fairly common, with random groups of people (such as otherwise healthy blood donors) having a rate of 1:600. Other disorders are distinctly more uncommon, with incidences between 1:100,000 and 1:2,000,000 being reported.