Background: Today, in Western countries, patients with allo-antibodies to high-frequency antigens or with complex antibody mixtures represent one of the most significant challenges in transfusion medicine. Another important aspect is the prevention of allo-immunization of patients who lack high-frequency antigens. In these conditions, the availability of a bank of a rare red blood cell group, supported by a database of donors subjected to extensive erythrocyte typing (preferably using erythrogenomic study), can constitute a resource of great value. Materials and Methods: Repeat Caucasian blood donors of group A or O, with selected Rh phenotypes (CCDee, ccDEE, ccdee, ccDee), aged under 52 years, were considered for typing. Moreover, we selected all non-Caucasian repeat blood donors for typing. For extended phenotyping and genotyping we adopted commercial methods supplied by Grfols and Werfen, respectively. For cryopreservation, we selected a high glycerol method in −80 °C electric freezer; blood unit processing was performed using a Haemonetics ACP 215 automated cell processor with close circuit devices. Results: We considered the five patients as follows: PA was massively transfused for a road trauma, developed multiple allo-antibodies (anti-D, anti-k), and required compatible blood units for an elective cardiac surgery; PB was a pregnant woman with anti-Coa (a high frequency antigen) monitored during pregnancy and in which it was necessary to proceed with the transfusion of the newborn; PC was a poly-transfused patient with myelo dysplastic syndrome who developed multiple allo-antibodies (anti-k, anti-CW, anti-Lea) and required continuous supportive therapy, including the procurement of compatible units and the implementation of therapeutic actions in an attempt to reduce the transfusion requirement using luspatercept; PD was a patient with sickle cell disease. They had a Fy (null) genotype, making it very difficult to find compatible units; and PE was interesting for the complexity of the immunohematological and erythrogenomic study performed to characterize a recipient with a rare phenotype and thus allow the transfusion of compatible units, preventing allo-immunization. Discussion: In this report, we have maintained a narrative approach. Starting with five patients representing as many clinical situations as possible, we have illustrated the approach followed for the immune-hematological study and the choices made to try to guarantee effective and safe transfusion therapy.