Ficolin-3 (also called H-ficolin or Hakata antigen) is a complement-activating 
pattern recognition molecule, possessing a fibrinogen-like domain involved in 
carbohydrate binding. Amongst human ficolins, Ficolin-3 has the highest 
concentration in serum and is the most potent lectin pathway activator in vitro. 
Evidence for its physiological function is sparse, although its deficiency has 
been suggested to increase susceptibility to infections. The specificity of 
Ficolin-3 is poorly characterized and currently few ligands are known. Here we 
report agglutination of Hafnia alvei, a Gram-negative enteric commensal 
bacterium and opportunist pathogen, in the presence of recombinant Ficolin-3 and 
calcium. Ficolin-3 also augmented phagocytosis of H. alvei by macrophages and 
displayed bactericidal activity. Additionally, Ficolin-3 inhibited host cells' 
response to TLR4/MD-2/CD14-LPS dependent NF-κB activation. This is the first 
demonstration of protective activity of Ficolin-3 against a human bacterial 
pathogen. Although human Ficolin-3 does not recognise and bind to common 
pathogenic bacteria, it could be an important component of innate immunity 
providing protection, for example, from commensal flora that can cause 
extraintestinal, opportunistic infections.