The spleen is an important organ in the defense of the body against pathogenic bacteria. Major functions of the spleen include antibody production and mechanical filtration of blood. Anatomically or functionally asplenic individuals are at increased risk of fulminant infection by encapsulated bacteria, particularly Streptococcus pneumoniae, Hemophilus influenzae, and Neisseria meningitidis. Polysaccharide vaccines are available against some strains of these pathogenic bacteria. More data are required to define specific age and risk groups. A search for better and more immunogenic vaccines, which may prove effective in a wider variety of patients, is currently under way. Although the current vaccines are not always effective and future revaccination may increase the incidence of adverse effects, most asplenic persons should receive the currently available vaccines to minimize their risk of life-threatening infection.