The role of the spleen in the immune response following naturally acquired exposure to encapsulated bacteria.

Female New Zealand White Rabbits following splenectomy (n = 9), splenectomy with 50% splenic autotransplantation (n = 8) and sham laparotomy (n = 9) have been serially exposed to type 2 Streptococcus pneumoniae and Haemophilus influenzae by aerosol inhalation. Animals were sampled for 3 weeks after exposure and the IgG and IgM type-specific antibody response measured by enzyme-linked immunosorbent assay. Haemophilus influenzae initiated a substantial anti-haemophilus IgG response which was not diminished by splenectomy. The anti-haemophilus IgM response was present in sham-operated animals, absent following splenectomy, and partially restored by splenic autotransplantation. Type 2 Streptococcus pneumoniae induced a minimal IgG and IgM antibody response in all animals irrespective of the presence or absence of a spleen. The results support the role of the spleen in mediating IgM production against polysaccharide encapsulated bacteria. The differential degree of immune response produced by the two organisms may explain in part the differential frequency with which these two organisms infect man following splenectomy.