Which organ absence markedly increases susceptibility to infections with encapsulated bacteria?

• A. Spleen
• B. Liver
• C. Thymus
• D. Lymph nodes

Answer: (A)

The spleen plays a pivotal role in defending against encapsulated bacteria because it filters the blood and mounts rapid antibody-mediated responses to polysaccharide capsules. Encapsulated organisms resist phagocytosis unless they are opsonized. IgM antibodies against the capsule promote complement activation, leading to C3b deposition that tagging the bacteria for phagocytosis. The spleen contains marginal zone B cells that specialize in producing these anti-capsular IgM antibodies quickly, and its macrophages in the red pulp efficiently clear opsonized bacteria from the bloodstream.

When the spleen is absent or nonfunctional (asplenia or functional asplenia), this opsonization and subsequent clearance are severely reduced, leaving individuals highly susceptible to serious infections by encapsulated bacteria such as Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis.

The thymus is mainly involved in T cell maturation, which is less directly linked to the immediate humoral response against encapsulated blood-borne bacteria. Lymph nodes contribute to many immune responses but are not the primary site tasked with clearing encapsulated bacteria from the bloodstream as the spleen is.