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IgA1
IgA1 Full Name
Immunoglobulin A1
IgA1 Introduction
Immunoglobulin A (IgA) is the second most abundant antibody in human serum and the predominant antibody class at mucosal surfaces, playing a crucial role in the body's immune defense system. IgA exists in two main subclasses: IgA1 and IgA2, which exhibit significant differences in structure, distribution, and function. IgA1 is the primary form of IgA found in serum, accounting for approximately 85-90% of total circulating IgA. In contrast, IgA2 predominates in mucosal secretions, such as those in the gut and respiratory tract, largely due to its greater resistance to degradation by microbial proteases. The most distinctive structural feature of the IgA1 molecule is its unique hinge region. Compared to IgA2, IgA1 has a longer hinge region composed of a proline-, serine-, and threonine-rich repetitive sequence. This segment contains 3 to 6 potential O-linked glycosylation sites. The glycosylation process in this hinge region is complex, and abnormalities at any stage can alter the sugar chain structure. Of particular interest is the absence of galactose, which results in the formation of so-called "Galactose-deficient IgA1" (Gd-IgA1). This structurally abnormal IgA1 molecule is considered the core pathogenic agent in the development of various diseases, most notably IgA Nephropathy (IgAN).
Figure 1. Schematic structure of IgA1, IgA2, and SIgA. (Source: Scheurer S, et al. 2023)
IgA1 is primarily produced by plasma cells in the bone marrow and mucosa-associated lymphoid tissue (MALT), with the gut-associated lymphoid tissue (GALT) being a significant source of pathogenic Gd-IgA1. At mucosal surfaces, secretory IgA forms the first line of defense against pathogen invasion. It functions through a mechanism known as "immune exclusion": it binds to invading bacteria, viruses, or toxins, prevents their adhesion to and invasion of epithelial cells, and facilitates the clearance of these complexes via mucus movement. This process is non-inflammatory and vital for maintaining mucosal homeostasis, as it effectively controls microbes without triggering a strong inflammatory response. In the circulatory system, serum IgA1 (mostly in monomeric form) participates in systemic immune responses. Although IgA1 is generally considered a weak activator of complement, particularly ineffective at activating the classical pathway—which helps it avoid causing excessive inflammatory damage while performing its immune functions—it can trigger a range of effector functions by binding to the IgA Fc receptor (FcαRI or CD89) expressed on various immune cells (e.g., neutrophils, monocytes/macrophages, eosinophils). These functions include antibody-dependent cellular cytotoxicity (ADCC), phagocytosis, respiratory burst, and the release of various cytokines and chemokines.
IgAN is the most common primary glomerular disease worldwide, pathologically characterized by the diffuse deposition of IgA1 immune complexes in the glomerular mesangium. Beyond IgAN, abnormal IgA1 is also closely linked to IgA vasculitis. IgA vasculitis is a systemic vasculitis affecting small blood vessels in the skin, joints, gastrointestinal tract, and kidneys. Its pathogenesis is highly similar to that of IgAN, also involving the deposition of immune complexes formed by Gd-IgA1 in vessel walls.
Alternate Names for IgA1
IgA1; Immunoglobulin A1
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