Asplenia refers to the absence of the spleen. It can be anatomically absent, or present but without normal functioning, a condition known as functional asplenia. Asplenia can be congenital, like when it fails to develop during embryological development. It can also be acquired, usually as a result of abdominal trauma, or surgical removal.
The main function of the spleen is to filter and process old red blood cells. It also offers immunological protection against encapsulated microorganisms (e.g. Streptococcus pneumonia, Haemophilus influenzae, Neisseria meningitidis, …). So, asplenia increases susceptibility to infections with encapsulated bacteria, such as pneumococcal pneumonia and meningitis.