C-Type Lectins from Marine Bivalves: Functional Diversity and Structural Insights.

C-type lectins (CTLs) are a large family of calcium-dependent carbohydrate-binding proteins that play crucial roles in innate immunity as pattern recognition receptors. Bivalve mollusks possess exceptionally diverse and expanded repertoires of CTLs, yet a systematic review integrating their structural, functional, and regulatory aspects has been lacking. This article provides a comprehensive synthesis of current knowledge on bivalve CTLs, analyzing their biosynthesis, complex tissue-specific expression under both normal and stressed conditions, and their multifaceted roles in immune defense and other physiological processes. Our analysis consolidates data on their diverse domain architectures, phylogenetic relationships, and the variability of key motifs within their carbohydrate-recognition domains. The results demonstrate that bivalve CTLs are not only critical for pathogen recognition, agglutination, and phagocytosis but also involved in processes like nutrition, development, byssus formation and biomineralization. However, a significant finding is that the detailed carbohydrate specificity for most bivalve CTLs remains poorly characterized, often limited to monosaccharide inhibition assays. In conclusion, while the immune role of bivalve CTLs is well-established, this review underscores a critical gap in understanding their fine glycan-binding profiles. Therefore, a shift in the focus of future research towards elucidating their structure and carbohydrate specificity is required for a full understanding of their biological functions and an assessment of their biomedical potential.