Does low immunoglobulin A weaken mucosal defense against fungi and other microbes?

Secretory immunoglobulin A (sIgA) is the primary immunological barrier on mucosal surfaces, crucial for maintaining host-pathogen homeostasis. In older adults, immunosenescent declines in salivary or intestinal sIgA secretion significantly compromise these mucosal defenses.

Mechanistic pathways of mucosal impairment

• Loss of immune exclusion: Under normal physiological conditions, sIgA binds to opportunistic pathogens to prevent them from adhering to epithelial cells, trapping them in mucus for clearance. A deficiency in sIgA removes this first line of defense.
• Fungal transformation and invasion: Secretory IgA normally suppresses the virulence of Candida albicans. Without sIgA, the fungus easily adheres to epithelial surfaces and undergoes a morphological transition into its highly invasive, virulent hyphal growth phase.
• Loss of metabolic suppression: Normal sIgA actively limits fungal growth by mechanisms such as the inhibition of fungal ergosterol biosynthesis. The absence of sIgA permits unchecked fungal proliferation and mucosal penetration.

Clinical and infectious implications

• Opportunistic pathogens: Reduced sIgA levels allow abnormal microbial infiltration, elevating the risk of oral, respiratory, and gastrointestinal infections.
• Localized dysbiosis: While compensatory mechanisms like IgM secretion may partially activate, they are generally insufficient to prevent localized dysbiosis and mucosal barrier disruption, which can subsequently trigger chronic inflammatory cascades.

Bottom line

• Low immunoglobulin A directly weakens mucosal defense by impairing immune exclusion, facilitating the invasive hyphal transformation of Candida albicans, and increasing vulnerability to opportunistic bacterial and fungal infections.