BACKGROUND The following parameters were analyzed: C5a, which is significant in classical and alternative pathways of the complement system activation, and factor H, the major function of which is to regulate the alternative pathway. Factor H, in the case of Borrelia infection, is combined by CRASPs proteins of spirochetes, and thus prevents C3b molecules from contact with the pathogen, opsonisation and lysis of bacteria. METHODS The experimental material in the model for the presented work consisted of whole blood of healthy persons (without the presence of antibodies anti-Borrelia) and persons with clinical symptoms of Lyme disease, which was stimulated with three genospecies of spirochetes recognized as pathogenic in Poland and Europe: B. afzelii, B. burgdorferi s.s. and B. garinii. RESULTS The increase in the level of C5a in the experimental model after in vitro stimulation of whole blood with three genospecies Borrelia can be treated as an indicator of an effective activation of the complement's cascade. The increase in level C5a in the plasma relies on the genospecies and the strongest is for B. garinii. The decrease in the level of factor H, observed after the incubation of whole blood with three genospecies Borrelia, shows that this parameter was included in the spirochetes' mechanisms acting against factors of the innate immunity system of a host, which prevents lysis of bacteria via the alternative pathway. CONCLUSIONS The results obtained on the basis of the in vitro model can be analysed from the aspect of spirochetes' real contact with a host's organism during the bite of infected ticks. Despite blocking of the alternative pathway, Borrelia initiate the activation cascade regardless of antibodies via the first contact of a host's organism with spirochetes, or in accordance with antibodies during the infection or subsequent contact with bacteria.