To assess B-cell function in patients under immunoglobulin (IgG)-replacement 
therapy, the non-licensed artificial bacteriophage (ΦX174)-neo-antigen may be 
used despite limited availability and experience. Active immunization against 
tick-borne encephalitis (TBE) is performed in few European countries. To test 
the feasibility of using licensed TBE vaccination as (neo-)antigen to determine 
residual or restored B-cell function in patients under regular IgG substitution, 
TBE-IgG levels were analyzed in 18 patients with ≥ 1-2 years of regular 
intravenous or subcutaneous IgG substitution and in pharmaceutical 
IgG-preparations (n=21 batches, 10 products). Six individuals were boosted 
against TBE. Although TBE-specific IgG was detectable in concentrates 
(281-57,100 VieU/0.5 μL), levels were only borderline in patient sera (n=31, 18 
individuals; median 132 VieU; positive >155). Thus, TBE vaccination may be used 
to test B-cell function under IgG replacement therapy because IgG substitution 
appears insufficient to yield protective TBE-specific antibody levels in 
children.