Simplify the following text for a Grade 7 native English-speaking person.
Original: A blood type ( also called a blood group ) is a classification of blood based on the presence or absence of inherited antigenic substances on the surface of red blood cells ( RBCs ) .
Simplified: It describes the type of blood a person has . This blood type is based on whether or not there are substances on the outside of red blood cells that can cause an allergic reaction when it comes into contact with certain other blood cells .