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Structure and biologic functions of IgA

Structure and biologic functions of IgA
Author:
Leman Yel, MD
Section Editor:
Luigi D Notarangelo, MD
Deputy Editor:
Anna M Feldweg, MD
Literature review current through: Nov 2022. | This topic last updated: Nov 29, 2022.

INTRODUCTION — Immunoglobulin A (IgA) is the most abundant type of antibody in the body, comprising most of the immunoglobulin in secretions and a significant amount of circulating immunoglobulin. In secretions, it serves to protect the mucosal tissues from microbial invasion and maintain immune homeostasis with the microbiota. The distribution, structure, production, biologic functions, and regulation of IgA will be discussed in this review.

Selective IgA deficiency is reviewed separately. (See "Selective IgA deficiency: Clinical manifestations, pathophysiology, and diagnosis" and "Selective IgA deficiency: Management and prognosis".)

DISTRIBUTION — IgA is the most abundant antibody isotype in the body, comprising almost 70 percent of the body's total immunoglobulin. The majority of IgA is found in the various mucous secretions, including saliva, milk, colostrum, tears, and secretions from the respiratory tract, genitourinary tract, and prostate [1-3].

Normal serum levels — IgA is the second most abundant isotype in the circulation, following immunoglobulin G (IgG) [4-8]. IgA levels, generally absent at birth, gradually increase throughout the first year of life to about 30 percent of adult levels at one year. Adult levels of IgA are reached in adolescence [9]. Normal serum levels range from 61 to 356 mg/dL [10].

Abnormal levels — Increased serum levels of IgA are seen in several inflammatory disorders, including IgA nephropathy, immunoglobulin A vasculitis (Henoch-Schönlein purpura), acquired immune deficiency syndrome (AIDS), alcoholic cirrhosis, advanced hepatitis, IgA myeloma, and several autoimmune diseases (eg, rheumatoid arthritis, systemic lupus erythematosus).

Decreased levels of serum IgA are seen in selective IgA deficiency, several other immunoglobulin deficiencies, and ataxia-telangiectasia and may be seen with acute and chronic lymphocytic leukemia, chronic myelogenous leukemia, macroglobulinemia, and heavy chain disease. (See "Selective IgA deficiency: Clinical manifestations, pathophysiology, and diagnosis", section on 'Differential diagnosis'.)

STRUCTURE — IgA was first described in 1953 [4]. Each IgA molecule is composed of two heavy chains and two light chains. Each heavy chain consists of one variable and three constant regions, whereas each light chain has one variable and one constant region. One monomeric IgA molecule has two Fab portions, which bind antigens (figure 1).

There are two subclasses of IgA monomer: IgA1 and IgA2. Each IgA subclass has slightly different constant heavy chains, which are encoded by two separate alpha-1 and alpha-2 genes on chromosome 14 [5-8]. The main structural difference between them is that IgA2 has a shorter hinge region compared with IgA1.

Both IgA1 and IgA2 carry a number of N-linked oligosaccharides (ie, glycans). In addition, the hinge region of IgA1 also harbors nine potential O-glycosylation sites, of which three to five are usually occupied by O-glycans, which may contribute to the limited conformational variability of IgA1 compared with IgA2 [11-13].

IgA is found in the body in two forms: monomeric and polymeric. Monomeric IgA circulates in the peripheral blood. The polymeric form is found in mucosal secretions of the respiratory, intestinal, and genitourinary systems, hence the name "secretory IgA" [5-8,14].

Monomeric serum IgA is mostly composed of IgA1 molecules. These have two Fab portions that bind antigens (figure 1). Once the Fab portions have bound antigen, the Fc portion binds to Fc-alpha receptors located on the cell surface of neutrophils, eosinophils, monocytes, macrophages, dendritic cells, and Kupffer cells [8,15]. IgA binding to this receptor initiates ingestion and destruction of the micro-organism by the phagocyte. (See "The adaptive humoral immune response", section on 'Opsonization'.)

IgA in its polymeric form is primarily dimeric IgA consisting of two monomeric IgA molecules linked to each other with a J (joining) chain and stabilized with a small molecule called the secretory piece (figure 1). This complex is referred to as "secretory IgA." The J-chain serves as a template for incorporation of monomers to form a stable IgA polymer, which has an amyloid-like assembly of the oligomerized structure. The J-chain also confers asymmetry for polymeric immunoglobulin receptor (pIgR) binding on the basolateral surface and transcytosis through the epithelium to the mucosa [16]. The antigen-binding capacity of the dimeric IgA is twice that of the monomer. Larger secretory IgA polymers (tetrameric or pentameric structures) have higher neutralizing potency, particularly for low affinity repetitive antigenic epitopes, such as those found on the surface of many bacteria and viruses. Trimeric, tetrameric, and larger forms, which have six to eight antigen-binding sites or more, have been shown in nasal secretions of healthy individuals [14]. Secretory IgA in the intestine is mostly composed of IgA2 molecules, whereas the IgA1 isotype dominates in the parotid gland [17]. The shorter hinge region of IgA2 enables secretory IgA to resist bacterial proteases in the lumen of the gastrointestinal system [17,18]. The secretory piece of the secretory IgA dimer is actually the secreted component of the polymeric immunoglobulin receptor, which is located on the basolateral surface of the mucosal epithelial cell [19,20]. The secretory piece protects the IgA dimer from being degraded by the proteolytic enzymes of the lumens. (See 'Production' below.)

Secretory IgA not only binds to bacterial antigens via specific recognition by means of Fab portions, but also coats some bacterial species through N- and O-glycan-mediated nonspecific innate interactions [11]. (See 'Production' below.)

PRODUCTION — Daily IgA production (serum plus secretory IgA combined) greatly exceeds production of all other immunoglobulins [21-25]. Approximately 75 percent of all produced immunoglobulin is IgA, primarily in the gut, milk, and bronchial secretions. Serum IgA is produced by plasma cells in the bone marrow. It is not clear where these plasma cells are originally generated or whether there is a contribution of mucosal plasma cells to the bone marrow compartment [25]. Secretory IgA is produced locally in the mucosal tissues and is not derived from serum IgA. In fact, serum IgA and secretory IgA are molecules with different biochemical and immunochemical properties produced by cells with different organ distributions [8]. In the gastrointestinal system, IgA originates from the follicular B cells of the gut-associated lymphoid tissue (GALT), which is composed of organized Peyer's patches, mesenteric lymph nodes, isolated lymphoid follicles, and nonorganized lamina propria. Further discussions of the mucosal immune system in the context of the development of food allergy and inflammatory bowel disease are found separately. (See "Pathogenesis of food allergy", section on 'The gut immune system' and "Immune and microbial mechanisms in the pathogenesis of inflammatory bowel disease", section on 'The mucosal immune system'.)

IgA production occurs by T cell-dependent mechanisms in Peyer's patches or mesenteric lymph nodes, as well as by T cell-independent mechanisms in the lamina propria [1-3,26]. Intestinal epithelial cells, dendritic cells, and local stromal cells may contribute to T cell-independent production of IgA locally by secreting thymic stromal lymphopoietin (TSLP), interleukin-6 (IL-6), interleukin-10 (IL-10), tumor necrosis factor-alpha (TNF-alpha), transforming growth factor-beta (TGF-beta-1), B cell-activating factor (BAFF), and a proliferation-inducing ligand (APRIL) [2,3]. A general discussion of antibody production is found elsewhere. (See "The adaptive humoral immune response".)

During development, intestinal B cells undergo VJ (for light chains) and VDJ (for heavy chains) gene somatic hypermutation (SHM) and class-switch recombination (CSR) from IgM to IgA, mainly in the germinal center of Peyer's patches and mesenteric lymph nodes. In the distal human gut, IgA2-producing plasma cells appear to develop in the lamina propria by direct CSR from IgM to IgA2 in a T cell-independent manner [27]. A T cell-independent pathway is also sufficient to coat most small intestinal microbes, resulting in increased uptake of microbes into Peyer's patches and thereby inducing the IgA production with a positive feedback [28]. (See "Immunoglobulin genetics".)

The IgA-secreting plasma cell, which has developed through T cell-dependent or T cell-independent pathways, migrates in the lamina propria to reach the proximity of the epithelial surface and releases IgA molecules. These IgA molecules bind to the polymeric immunoglobulin receptor on the basolateral surface of the mucosal epithelium. This same receptor is involved in the transport of pentameric IgM across epithelial surfaces. Bound IgA and immunoglobulin receptor complex is internalized into the cell and transported to the apical surface of the epithelium where the IgA molecule dissociates from the immunoglobulin receptor and is secreted into the lumen carrying a part of the immunoglobulin receptor, which is called the secretory piece. Dimeric IgA molecules bind antigens in the gut and are expelled.

FUNCTIONS — IgA appears to be important in immune functions, although it has intrigued investigators for years that many patients with IgA deficiency do not experience more frequent or severe infections [7,17]. This disconnect between the immunologic role of IgA and clinical observations in individuals with IgA deficiency is presumed to be attributable to redundant immunologic mechanisms that protect the host from microbial invasion. Specifically, secretory IgM may perform many of the same functions as IgA and may somewhat compensate for lack of IgA in normal neonates and in patients with IgA deficiency [29]. Selective IgA deficiency and possible compensation by IgM are discussed in detail separately. (See "Selective IgA deficiency: Clinical manifestations, pathophysiology, and diagnosis" and "Selective IgA deficiency: Management and prognosis".)

Serum IgA — The function(s) of serum IgA is not fully understood. One important function appears to be activation of the phagocytic system through the binding of the Fc portion to the cell surface receptors [2,8,12]. Serum IgA binds to Fc-alpha-RI (CD89), the only Fc receptor monospecific for IgA on granulocytes, monocytes, some dendritic cells, and macrophages, resulting in clearance of immune complexes (formed by foreign antigens and IgA) from the circulation by the phagocytic system [30]. Binding of IgA to Fc-alpha-RI can also lead to cellular inhibition, thus Fc-alpha-RI acts as a regulator of cellular responses and may play a role in different inflammatory diseases, such as IgA nephropathy, immunoglobulin A vasculitis (Henoch-Schönlein purpura), and dermatitis herpetiformis [31].

Serum IgA does not fix complement or activate the classic complement pathway and therefore does not have a considerable role in complement-mediated effector immune functions. The lack of complement activation means that this mechanism can clear antigens from the circulation without generating significant inflammation [30,32]. However, interaction of IgA with Fc-alpha-RI can lead to anti-, non-, or proinflammatory responses, depending on the environment. Binding of monomeric serum IgA to Fc-alpha-RI, without cross-linking of the receptor, causes phosphorylation of an immunoreceptor tyrosine-based activation motif (ITAM), which in turn induces the recruitment of the tyrosine phosphatase SHP-1 (ie, Src homology region 2 domain-containing phosphatase-1) to Fc-alpha-RI, leading to deactivation of several activating pathways of the immune system [33]. On the other hand, cross-linking of Fc-alpha-RI by IgA that has opsonized a bacterium results in induction of proinflammatory cellular functions, such as phagocytosis, antibody-dependent cellular cytotoxicity (ADCC), respiratory burst, degranulation, antigen presentation and release of cytokines and inflammatory mediators [31,34].

With the balanced IgA effects, inflammatory reactions and autoimmune processes are prevented in normal individuals. On the contrary, in IgA-deficient patients, Fc-alpha-RI inhibitory signaling would be eliminated because of the absence of IgA binding with Fc-alpha-RI, resulting in a predisposition to autoimmune processes [30].

Secretory IgA — Secretory IgA is the most abundant immunoglobulin in mucous secretions. Mucosal membranes in the body cover an approximate area of 200 to 400 m2, harboring an estimated 15,000 to 36,000 species and 1800 genera of microbiota [12,35-38]. Secretory IgA coats endogenous bacteria in the oral cavity, intestinal tract, respiratory, and genital tracts. This limits the epithelial adherence and penetration of these bacteria and confines the bacteria to the mucosal surfaces [37].

Features of innate and adaptive immunity — Secretory IgA appears to function as part of both the adaptive and innate immune systems [11,17,36,37,39]. Certain features, like the ability of the Fab sites to bind specific antigens and undergo somatic hypermutation (SHM) to increase their affinity for those particular antigens, are characteristic of the adaptive immune response. Through the Fab sites, IgA antibodies bind bacterial antigens and interfere with bacterial adherence to epithelial cells, thereby protecting mucosal epithelial cells from specific bacterial pathogens [19]. Secretory IgA can also bind and neutralize viruses or toxins, such as Escherichia coli or Shigella enterotoxin, and inflammatory microbial molecules, such as lipopolysaccharide (LPS) [2,3,26,30,32,35-37,40].

Other features of secretory IgA are more characteristic of the innate immune system. For example, the N- and O-glycans are similar to the glycans on microbial surfaces and appear to act as decoys, binding to receptors for these molecules on the luminal surfaces of intestinal epithelial cells, thus preventing bacterial attachment. In addition, N- and O-glycans are probably functional in coating of the numerous and diverse bacteria indigenous to the oral cavity and intestinal tract.

Intestinal homeostasis — Secretory IgA appears to be central to intestinal homeostasis between the host and commensal bacteria. Most of the commensal bacteria within the body are located in the gastrointestinal tract [1-3,26]. IgA is probably critical in regulating bacterial communities, favoring commensal organisms in biofilms and preventing pathogen overgrowth.

This has been demonstrated in activation-induced cytidine deaminase (AID) knockout mice that lack secretory IgA, in which excessive expansion of anaerobic bacteria in the entire proximal intestinal system can develop [41,42]. Dysbiosis of the intestine has also been observed in another AID gene-targeted mouse model, in which SHM is disrupted while class-switch recombination (CSR) is preserved [43]. Aberrant anaerobic expansion is found in other murine models with IgA deficiency (eg, recombination-activating gene 2 [RAG2] knockouts, severe combined immunodeficiency [SCID] models) [44]. Similarly, some patients with selective IgA deficiency or common variable immunodeficiency (CVID) develop small bowel bacteria overgrowth syndrome, resulting in various clinical manifestations of the intestine [2]. This type of change in the intestinal microbial ecology may cause activation of mucosal immune cells, including intraepithelial lymphocytes, cells of the isolated lymphoid follicles, Peyer's patches, and mesenteric lymph nodes. In addition, this inflammatory state may become systemic and involve lymphocytes of all germinal centers and lymphoid tissues.

The inability of secretory IgA to fix complement and stimulate the release of inflammatory mediators may also play a role in creating a noninflammatory host microbial relationship [33,45]. Consistent with this concept, patients with selective IgA deficiency, as well as those with other primary antibody deficiencies, such as CVID and hyperimmunoglobulin M syndromes, tend to develop inflammatory bowel disease in addition to gastrointestinal infections [2]. Patients with selective IgA deficiency or CVID may also develop intestinal nodular lymphoid hyperplasia, which may be due to polyclonal activation of intestinal B cells by bacterial overgrowth (picture 1). (See "Selective IgA deficiency: Clinical manifestations, pathophysiology, and diagnosis", section on 'Gastrointestinal disorders (noninfectious)' and "Selective IgA deficiency: Clinical manifestations, pathophysiology, and diagnosis", section on 'Gastrointestinal' and "Clinical manifestations, epidemiology, and diagnosis of common variable immunodeficiency in adults", section on 'Gastrointestinal disease'.)

Secretory IgA in milk — Secretory IgA is the most important immunoglobulin in human breast milk [46]. It is produced by maternal plasma cells in the mammary gland and is directed against microbial structures encountered on the mucosal surfaces of the mother [47]. The IgA concentration peaks in the colostrum and remains high until seven to eight months of age, with a gradual decrease in mature milk [48]. IgA concentration is particularly high in the breast milk of mothers who have preterm newborns, to meet the demands of the infant [49]. Secretory IgA antibodies in milk are of broad spectrum with the ability to recognize different types of both indigenous and pathogenic micro-organisms and dietary allergens. Secretory IgA binds to those antigens in the intestinal lumen of the infant, protects from various infections, and also modulates the long-term establishment of the commensal intestinal microbiota [43]. In addition, secretory IgA may reduce the allergenicity of dietary antigens by binding to them [46]. The IgA fraction also contains natural autoantibodies that enhance the immune response and probably modulate development of the entire immune system of the infant [50]. (See "Infant benefits of breastfeeding".)

REGULATION — IgA production is regulated by the molecules that influence somatic hypermutation (SHM) and class-switch recombination (CSR). These molecules include receptors that mediate T and B cell interaction (CD40, CD40L), molecules important in receptor editing (eg, activation-induced cytidine deaminase [AID]), and cytokines, such as transforming growth factor-beta (TGF-beta), interleukin-6 (IL-6), and interleukin-10 (IL-10). SHM and CSR are critical to the various functions of IgA [2,3]. Diversification of IgA through SHM is critical to maintaining a balanced commensal bacterial population in the gut. CSR from IgM (the most efficient complement fixing immunoglobulin isotype) to IgA (which does not fix complement), contributes to a hypoinflammatory state within the intestine, as discussed previously. (See 'Intestinal homeostasis' above.)

The inhibitory coreceptor programmed cell death-1 (PD-1) in mice has been shown to affect the B cell germinal center dynamics by controlling the number and nature of T helper cells in the Peyer's patches with an impact on selection of IgA plasma cells and ultimately, the bacterial composition of the gut [51,52]. Another regulator of secretory IgA production appears to be T helper type 17 (Th17) cells in mice [53,54]. It has been demonstrated that Th17 cells increase polymeric immunoglobulin receptor expression in the bronchial epithelium in response to inhaled antigen [53] and also upregulate intestinal polymeric immunoglobulin receptor and IgA, thereby contributing to gut homeostasis [54]. In addition, Th17 cells have been shown to acquire a follicular helper T cell phenotype and induce the development of IgA-producing germinal center B cells in Peyer's patches, resulting in high affinity T cell-dependent IgA production [55].

IgA-BASED THERAPY — There are no IgA preparations available for passive immunization in clinical practice. However, mucosal administration of polyclonal secretory IgA would be expected to enhance the immune response against a broad spectrum of mucosal pathogens, and research on IgA antibodies administered via nasal, oral, or intramuscular routes to provide passive infection protection is ongoing. In one report, recombinant monoclonal IgA antibody against the A/California/04/2009 (H1N1) virus administered intramuscularly prevented virus transmission in guinea pigs [56].

Purification and production of large amounts of secretory IgA is a major challenge. Promisingly, one report showed that polymeric IgA and IgM antibodies obtained from plasma were able to associate with recombinant secretory component molecules ex vivo, resulting in functional secretory-like antibodies and suggesting a potential for development of secretory IgA- and IgM-based mucosal therapies [57].

However, monomeric IgA as a therapeutic antibody isotype is receiving more attention because of its ability to recruit effector cells different from those recruited by IgG molecules, such as polymorphonuclear cells, and also to activate monocytes and macrophages [43,58]. There is interest in designing IgGA cross-isotype antibodies with an Fc domain that displays both IgA-like and IgG-like effector functions [58,59]. It is likely that IgA isotype and IgGA cross-isotype antibodies will be used in the future for the treatment of infectious, autoimmune, and malignant diseases.

SUMMARY

Immunoglobulin A (IgA) is the most abundant antibody isotype, comprising almost 75 percent of the body's total immunoglobulin. The majority of IgA is found in the various mucous secretions of the respiratory, intestinal, and genitourinary systems. Normal serum levels range from 61 to 356 mg/dL. (See 'Distribution' above.)

IgA has two forms: monomeric and polymeric (mostly dimeric) (figure 1). IgA circulates in the blood in monomeric form (ie, serum IgA). It is mostly in dimeric form (ie, secretory IgA) in mucosal secretions. (See 'Structure' above.)

Immunologic functions of IgA include protection from microbial invasion, intestinal homeostasis with commensal organisms and immune modulation, and dampening of inflammatory pathways that could lead to autoimmune processes:

Serum IgA binds to monocytes and granulocytes and clears immune complexes from the circulation. It can do so without activating complement or generating significant inflammatory signals, which is believed important in dampening inflammatory processes that lead to the formation of autoantibodies. (See 'Serum IgA' above.)

Secretory IgA helps protect mucosal surfaces from microbial invasion by coating microbes to prevent adherence to epithelial cells and by neutralizing microbial toxins and inflammatory molecules, such as lipopolysaccharide (LPS). However, there are apparently other overlapping mechanisms for this purpose because many IgA-deficient individuals do not suffer from excessive infections. (See 'Features of innate and adaptive immunity' above.)

Secretory IgA promotes intestinal homeostasis between the host and commensal bacteria by regulating bacterial communities, favoring commensal organisms in biofilms, and preventing pathogen overgrowth. Because IgA can function without activating complement or stimulating the release of inflammatory mediators, it is likely critical for the creation of noninflammatory host microbial interactions. (See 'Intestinal homeostasis' above.)

Secretory IgA in breast milk protects the infant from infection and contains natural autoantibodies that enhance the immune response and probably modulate development of the immune system. (See 'Secretory IgA in milk' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges E Richard Stiehm, MD, who contributed as a Section Editor to an earlier version of this topic review.

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