From MedlinePlus Genetics
Isolated congenital asplenia is a condition in which affected individuals are missing their spleen (asplenia) but have no other developmental abnormalities. While most individuals with this condition have no spleen at all, some people have a very small, nonfunctional spleen (hyposplenism).

The spleen plays an important role in the immune system. This organ is part of the lymphatic system, which produces and transports fluids and immune cells throughout the body. The spleen produces certain immune system cells called phagocytes that help remove bacteria from the blood in order to prevent infections. The spleen also stores particular blood cells that fight foreign invaders until they are needed and filters old blood cells for removal. Because people with isolated congenital asplenia lack these immune functions, they are highly susceptible to bacterial infections.

People with isolated congenital asplenia are prone to developing severe, recurrent infections. Infections most commonly affect the whole body (sepsis), the membrane covering the brain and spinal cord (meningitis), or the ears (otitis media). Infections are most often caused by the Streptococcus pneumoniae bacteria.

Without preventative care and proper treatment, the frequent infections caused by isolated congenital asplenia can be life-threatening.  https://medlineplus.gov/genetics/condition/isolated-congenital-asplenia