<P> Traumas, such as a road traffic collision, can cause rupture of the spleen, which is a situation requiring immediate medical attention . </P> <P> Human spleens have been noted to decrease in volume up to 40% when subjected to stimuli from strenuous exercise or hypoxic gas inhalation . </P> <P> Asplenia refers to a non-functioning spleen, which may be congenital, damaged by trauma, or caused by disease such as sickle cell anaemia . Hyposplenia refers to a partially functioning spleen . These conditions may cause a modest increase in circulating white blood cells and platelets, a diminished response to some vaccines, and an increased susceptibility to infection . In particular, there is an increased risk of sepsis from polysaccharide encapsulated bacteria . Encapsulated bacteria inhibit binding of complement or prevent complement assembled on the capsule from interacting with macrophage receptors . Phagocytosis needs natural antibodies, which are immunoglobulins that facilitate phagocytosis either directly or by complement deposition on the capsule . They are produced by IgM memory B cells (a subtype of B cells) in the marginal zone of the spleen . </P> <P> A splenectomy (removal of the spleen) also causes asplenia and results in a greatly diminished frequency of memory B cells . A 28 - year follow - up of 740 World War II veterans whose spleens were removed on the battlefield showed a significant increase in the usual death rate from pneumonia (6 rather than the expected 1.3) and an increase in the death rate from ischemic heart disease (41 rather than the expected 30), but not from other conditions . </P>

Which one of the following is not a function of the spleen