Ficolin-1 (M), ficolin-2 (L), ficolin-3 (H) and mannan-binding lectin (MBL) 
activate the complement system and have opsonic activity. The specificity of 
ficolin-3 is poorly characterized and currently limited to a few ligands only. 
We present new specific targets for human ficolin-3, identified among 
lipopolysaccharides (LPSs, endotoxin) of Hafnia alvei. The interaction was 
restricted to LPSs of four strains: 23, Polish Collection of Microorganisms 
(PCM) 1200, PCM 1203 and PCM 1205 and limited to their O-specific 
polysaccharides (O-specific PSs) composed of different numbers of 
oligosaccharide (OS) repeating units (RUs). Moreover, these LPS/ficolin-3 
complexes activated the lectin pathway of complement in a C4b-deposition assay 
in a calcium- and magnesium-dependent way. A neoglycoconjugate of the O-specific 
PS fraction of H. alvei 1200 LPS with bovine serum albumin (BSA) was prepared 
and used as a tool for the determination of ficolin-3 concentration and activity 
in serum. To confirm a structure of the O-specific PS 1200 selected for the 
conjugate preparation, structural analysis was performed on a series of 
O-specific PSs released by the mild acid hydrolysis of the LPS. The isolated 
O-specific PSs, showing the different length distributions, were devoid of a 
major part of the core OS region and had Hep-Kdo disaccharide at a reducing end. 
The neoglycoconjugate was a highly selective tool for the determination of 
ficolin-3 concentration and activity in serum (lectin pathway activation in the 
C4b deposition assay) and was not affected by MBL, ficolin-1 and ficolin-2 or 
natural antibodies.