Patients without a spleen (asplenia) experience an increased risk for septicaemia from encapsulated bacteria, which is associated with a high mortality rate. Meningococcal bacteria can cause such infections and serogroup B is the dominant meningococcal subtype in Europe. Therefore, vaccination for risk populations like patients without a spleen is a pressing matter. Considering the effectiveness of the meningococcal serogroup B vaccine, data for this high-at-risk population is currently lacking. The aim of this study is to evaluate the meningococcal B vaccine (BEXSERO®) in patients without a spleen compared to a healthy control group. A total of 40 patients and 40 healthy persons will receive a two-dose schedule of BEXSERO® with a one-month interval between doses. The effectiveness of the vaccine will be determined by measuring antibodies against different meningococcal strains in the blood of the patient. The amount of antibodies one month after second vaccination will be compared between patients and healthy persons. The most reliable assay to determine antibodies against meningococcal strains is the human serum bactericidal assay which will be carried out in a reference laboratory. Other end points are the persistence of antibodies after six months and the cellular immune response. The cellular immune response will be assessed by measuring the proliferation of certain immune cells like lymphocytes and the amount of produced cytokines (signalling proteins) after vaccination. In addition, the safety of the vaccine will be evaluated by documenting all adverse reactions to the vaccine. Overall, this study will be the first to assess the effectiveness of the meningococcal B vaccine in this high-at-risk population and provide data for vaccination guidelines.