Why can type O blood be donated to anyone?

People with type O blood are often called "universal donors" because their red blood cells can be safely transfused to people with any other blood type. This is because type O red blood cells do not have any A or B antigens on their surface, which means that they will not be recognized as foreign by the immune system of a recipient with a different blood type.

In contrast, people with type A, B, or AB blood can only donate blood to people with the same blood type or a compatible blood type. For example, people with type A blood can only donate blood to people with type A or AB blood, and people with type B blood can only donate blood to people with type B or AB blood.

The exception to this rule is when a person is receiving a stem cell transplant. In this case, the donor and recipient must have compatible blood types, but it is not necessary for the donor to have type O blood. This is because stem cells can develop into any type of blood cell, so they will eventually be able to produce red blood cells of the recipient's blood type.