Editing Overwhelming post-splenectomy infection (section)

==Mechanism==
The [[spleen]] contains many [[macrophage]]s (part of the [[reticuloendothelial system]]), which are immune cells that [[phagocytose]] (eat) and destroy [[bacteria]].  In particular, these macrophages are activated when bacteria are bound by [[Immunoglobulin G|IgG]] [[antibodies]] (IgG1 or IgG3) or the [[Complement system|complement]] component C3b.  These types of antibodies and complement are immune substances called [[opsonizer]]s, molecules that bind to the surface of bacteria to facilitate [[phagocytosis]].

When the spleen is no longer present ([[asplenia]]), IgG and C3b are still bound to bacteria, but they cannot be removed from the [[blood]] circulation due to the loss of the splenic macrophages.  Hence the bacteria are free to cause [[infection]].

Patients without a spleen often need [[immunization]]s against [[pathogen]]s that normally require [[opsonization]] and phagocytosis by macrophages in the spleen.  These include common human pathogens with [[bacterial capsules]] (''[[Streptococcus pneumoniae]], [[Salmonella typhi]], [[Neisseria meningitidis]], [[E. coli]], [[Hemophilus influenzae]], [[Streptococcus agalactiae]], [[Klebsiella pneumoniae]], [[Pseudomonas aeruginosa]]'').  Capsules made of [[polysaccharide]]s (sugars) permit bacteria to evade phagocytosis by macrophages alone, since only [[protein]]s are directly recognized by macrophages in phagocytosis.  So [[humoral]] immunity in forms of IgG and complement proteins is the [[human immune system]]'s response against bacterial capsules.