This chapter describes immune reactions occurring during transfusion of blood or blood products and adverse drug reactions. Coagulopathy, hypothermia, citrate toxicity hyperkalaemia and transfusion-related acute lung injury (TRALI) are some of adverse effects from massive blood transfusion. Anaphylaxis in obstetrics may be life threatening not only for the pregnant mother but also for the fetus. If anaphylaxis occurs at the time of a caesarean section, then the diagnosis may be difficult to make. There might be some confusion identifying the offending agent because several drugs are administered at the same time. Most reactions occur either due to an exaggeration of the predicted pharmacological effect or unpredictably and unrelated to a drug's main pharmacological action. Cyclizine, oxytocin, carboprost and atracurium are examples of drug that have an adverse effect when administered in the obstetric setting.