See below. Splenic infarction (cut off of blood supply to the spleen) can occur (this is painful!) when patients are kids. As patients enter adulthood, the spleen can eventually disappear. This can make patient more suspectible to infections that would not occur in patients with functioning spleens.
Decrased function. The spleen can slowly lose function over time, due to infarction from sickle cells. This is typically asymptomatic. It can also become swollen quickly, which is painful and may make you pretty sick. The loss os splenic function can increase the chance of infection, so fevers are concerning.