RegIIIγ and its human counterpart, HIP/PAP, are secreted C-type lectins that directly bind peptidoglycan on Gram-positive bacteria and exhibit antimicrobial activity. These proteins are expressed in the intestinal epithelium in response to microbial colonization, particularly by Gram-positive bacteria. The study shows that RegIIIγ and HIP/PAP are evolutionarily primitive forms of lectin-mediated innate immunity, playing a key role in maintaining symbiotic relationships between the host and gut microbiota. They help sequester bacteria in the intestinal lumen, preventing microbial invasion and inflammation. RegIIIγ is primarily expressed in Paneth cells of the small intestine, where it is secreted into the gut lumen. It binds to Gram-positive bacteria, including Listeria and Enterococcus species, and exhibits direct antibacterial activity. This activity is mediated through interactions with peptidoglycan, a component of Gram-positive bacterial cell walls. RegIIIγ and HIP/PAP also bind to chitin, a polysaccharide similar to peptidoglycan, and show high-affinity binding to peptidoglycan. These proteins do not require calcium for binding and are pattern-recognition molecules that recognize microbial-associated molecular patterns. Their antibacterial activity is inhibited by peptidoglycan and chitin fragments, linking peptidoglycan binding to antimicrobial function. RegIIIγ expression is upregulated in response to microbial colonization, with higher expression in the distal regions of the small intestine. It is also increased during postnatal development, particularly during weaning, when the gut microbiota changes. The study suggests that RegIIIγ and HIP/PAP are part of a compensatory response to maintain mucosal homeostasis in the face of changing microbial ecology. These proteins represent a new family of inducible antibacterial proteins that seek out microbial targets via interactions with bacterial peptidoglycan. They reveal a new function for mammalian C-type lectins and may have evolved from a directly antimicrobial precursor. The findings highlight the importance of RegIIIγ and HIP/PAP in innate immunity and suggest that their expression is regulated by distinct mechanisms compared to other antimicrobial proteins. The study provides new insights into how symbiotic host-microbial relationships are maintained and contributes to a better understanding of inflammatory bowel disease pathogenesis.RegIIIγ and its human counterpart, HIP/PAP, are secreted C-type lectins that directly bind peptidoglycan on Gram-positive bacteria and exhibit antimicrobial activity. These proteins are expressed in the intestinal epithelium in response to microbial colonization, particularly by Gram-positive bacteria. The study shows that RegIIIγ and HIP/PAP are evolutionarily primitive forms of lectin-mediated innate immunity, playing a key role in maintaining symbiotic relationships between the host and gut microbiota. They help sequester bacteria in the intestinal lumen, preventing microbial invasion and inflammation. RegIIIγ is primarily expressed in Paneth cells of the small intestine, where it is secreted into the gut lumen. It binds to Gram-positive bacteria, including Listeria and Enterococcus species, and exhibits direct antibacterial activity. This activity is mediated through interactions with peptidoglycan, a component of Gram-positive bacterial cell walls. RegIIIγ and HIP/PAP also bind to chitin, a polysaccharide similar to peptidoglycan, and show high-affinity binding to peptidoglycan. These proteins do not require calcium for binding and are pattern-recognition molecules that recognize microbial-associated molecular patterns. Their antibacterial activity is inhibited by peptidoglycan and chitin fragments, linking peptidoglycan binding to antimicrobial function. RegIIIγ expression is upregulated in response to microbial colonization, with higher expression in the distal regions of the small intestine. It is also increased during postnatal development, particularly during weaning, when the gut microbiota changes. The study suggests that RegIIIγ and HIP/PAP are part of a compensatory response to maintain mucosal homeostasis in the face of changing microbial ecology. These proteins represent a new family of inducible antibacterial proteins that seek out microbial targets via interactions with bacterial peptidoglycan. They reveal a new function for mammalian C-type lectins and may have evolved from a directly antimicrobial precursor. The findings highlight the importance of RegIIIγ and HIP/PAP in innate immunity and suggest that their expression is regulated by distinct mechanisms compared to other antimicrobial proteins. The study provides new insights into how symbiotic host-microbial relationships are maintained and contributes to a better understanding of inflammatory bowel disease pathogenesis.