Why is it important to know a person's rhesus factor before a blood transfusion?

Rh factor matters because an Rh-negative person can develop antibodies after exposure to Rh-positive blood. To avoid this reaction, an Rh-negative patient is generally given Rh-negative blood. An Rh-positive patient can usually receive either Rh-positive or Rh-negative blood, provided the other compatibility requirements are met.

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