Asplenia is a big word that means that somebody doesn't have a spleen. The spleen is a special organ inside your body that helps your immune system fight off germs and infections. When someone is born without a spleen, or if they have to have it removed because of a medical condition, they don't have that extra help from their immune system.
Without a spleen, it's important for the person to take extra care to protect themselves from getting sick. They might need to get special vaccines, like one called the pneumococcal vaccine that helps fight off a type of bacteria that can make them really sick. They also might have to take antibiotics to help prevent infections.
If someone with asplenia does get sick, they might need to go to the hospital right away, because they can get very sick very quickly. It's important for them to tell their doctor that they don't have a spleen so that they can get the right treatment.