The NHS maintain a regular supply of all blood types to ensure the right blood is available to everyone who needs it. They need enough of the right types of blood to meet day-to-day patient needs and to cope with emergencies.
Some blood types are in higher demand from hospitals than others. For example, it’s estimated that around 8% of people have O negative blood, but O negative makes up 13% of requests from hospitals.
This is because anyone can receive the red cells from O negative donors. So O negative red cells can be used in emergencies or when a person’s blood type in unknown.
Sometimes it is necessary to target specific blood types to increase stock levels. That’s why sometimes, regular donors with the particular blood type needed are contacted, and asked to give blood.