Infectious diseases are a global problem and about a third of total deaths are related to infections. Despite the successful development of antimicrobials in the 20th century, there has been a dramatic increase in antibiotic resistance in recent years. In order to develop new prophylactic methods and treatment strategies, it is essential to understand how pathogenic bacteria can manipulate the immune system.Any successful pathogen must avoid and defend itself against the complement system, a key component of the innate immune system, and develop mechanisms that allow the microbe to survive, colonize and, in the worst case, invade the host. The overall goals of the project are to increase the knowledge of the underlying mechanisms of microbial survival and find new therapeutic opportunities. We will highlight how bacteria can avoid attacks from the complement system by hijacking human or producing their own complement inhibitors and how they use complement receptors to attach to and invade epithelial cells. We will focus on respiratory pathogens such as streptococci. In addition, studies are planned to investigate the role of periodontal bacterial pathogens in the development of rheumatoid arthritis as such association is evident in epidemiologic studies.