Secretory IgA and Mucosal Antibody Transport

Secretory immunoglobulin A is the dominant antibody at mucosal surfaces and the most abundantly produced antibody class in the body. It is generated by plasma cells beneath the epithelium as dimeric IgA, then actively transported across the epithelial cell layer and released into secretions, where it defends the mucosa without provoking inflammation.

Definition

Secretory IgA is a dimeric, J-chain-containing immunoglobulin A complexed with secretory component that is transported across mucosal epithelium by the polymeric immunoglobulin receptor and released into external secretions, where it neutralizes pathogens and toxins and shapes the microbiota.

Scope

This topic covers the structure of secretory IgA, its transport across the epithelium by the polymeric immunoglobulin receptor, and its protective and homeostatic functions in mucosal secretions. It is a reference entry in mucosal immunology and does not provide clinical guidance.

Core questions

• How is secretory IgA structurally different from serum IgA?
• How is dimeric IgA transported across the epithelium into secretions?
• How does secretory IgA protect the mucosa without causing inflammation?
• How does secretory IgA influence the resident microbiota?

Key concepts

• Dimeric IgA and J chain
• Polymeric immunoglobulin receptor (pIgR)
• Secretory component
• Transcytosis across epithelium
• Immune exclusion
• Non-inflammatory neutralization
• Microbiota shaping by IgA

Mechanisms

Plasma cells in the mucosal lamina propria secrete IgA joined by the J chain into dimers. The polymeric immunoglobulin receptor on the basolateral surface of epithelial cells binds dimeric IgA and transcytoses it to the apical surface, where the receptor's extracellular portion is cleaved and remains attached as secretory component, releasing secretory IgA into the lumen. There, secretory IgA performs immune exclusion, binding and aggregating pathogens and neutralizing toxins so they cannot reach the epithelium, and it helps organize and contain the microbiota. Because IgA poorly activates inflammatory effector pathways, this defence is largely non-inflammatory, consistent with the tolerant character of mucosal immunity.

Clinical relevance

Secretory IgA provides front-line protection against enteric and respiratory pathogens, and IgA responses are a goal of mucosal vaccination; selective IgA deficiency is the most common primary antibody deficiency. This entry describes the underlying immunology for reference and is not a basis for individual diagnosis or treatment.

History

Secretory IgA was distinguished from serum immunoglobulins once secretions were found to contain a structurally distinct antibody bearing secretory component. Subsequent work defined the polymeric immunoglobulin receptor and the transcytosis pathway and clarified the dual protective and homeostatic roles of secretory IgA in shaping host-microbiota relationships.

Key figures

• Per Brandtzaeg
• Nicholas Mantis
• Oliver Pabst
• Blaise Corthesy

Seminal works

• brandtzaeg-2008
• mantis-2011
• pabst-2012

Frequently asked questions

What is secretory component?

Secretory component is the part of the polymeric immunoglobulin receptor that remains attached to IgA after it is transported across the epithelium; it stabilizes secretory IgA in the harsh mucosal environment and is part of what distinguishes secretory from serum IgA.

Why does secretory IgA not cause inflammation?

IgA is a poor activator of complement and inflammatory effector mechanisms, so it can bind and exclude pathogens and shape the microbiota largely without triggering the inflammation typical of other antibody classes, fitting the tolerant nature of mucosal surfaces.