Alloimmunization is described as an immune response to the recognition of a foreign antigen, causing the immune system to initiate the process of destruction of erythrocytes and formation of irregular antibodies. A total of 354 erythrocyte antigens have already been classified, divided into 44 systems, but not all of them are classified as clinically significant in transfusion practice. The ABO, RH, Kell, Duffy, Kidd and MNS systems are the most important, as they have the potential to cause intravascular or extra-vascular hemolysis. The RH system is of great clinical interest due to its high immunogenicity and high capacity for immune-mediated erythrocyte destruction.

To identify the frequency of the main antibodies of the RH system and to evaluate the profile of sensitized patients.

A retrospective study was conducted from January 2015 to May 2019. The population evaluated were individuals with positive irregular antibody research known as: Anti -D, C, c, E, e. Patients with irregular antibodies, identification of antibodies from other erythrocyte systems and newborn patients with antibodies of probable maternal origin were excluded from the study. Data were analyzed using descriptive statistics to assess the frequency of qualitative variables: age group, gender and antibody found.

An amount of 667 samples from patients with irregular antibodies were analyzed, and 30% of the sensitization was caused by antigens of the RH system. Regarding identification, the most frequent isolated antibody was Anti-E (38.6%), followed by Anti-D (30.7%), Anti-c (5.3%), Anti-C (4.8%) and Anti-e (1.1%). Antibody associations were also found, the most frequent being Anti -D and Anti -C (13.2%), Anti - E and Anti- c (3.7%) and other associations in less frequency (2.6%,). It was possible to observe that alloimmunization is a more frequent condition in patients aged over 60-years (58.2%), and the percentage of women is also higher (72.4%).

Through the presented data, it was possible to observe that alloimmunization is a more frequent event in patients over 60-years of age, which is justifiable, since the data analyzed showed that patients in this age group have more transfusions when compared to the other groups in this study. In addition, women also had a higher percentage of sensitization, as the event could be triggered by a transfusion or by secondary immune responses due to the probability of maternal-fetal immunoerythrocyte reactions from previous pregnancies, however, the gestational history was not analyzed during the research.

The alloimmunizations by erythrocyte antigens of the RH system represent approximately one third of the alloimmunizations within the analyzed period. Among the antigens of this system, the highest frequency of antibodies identified alone was Anti-E, followed by Anti-D. The highest number of sensitized patients was over 60-years old; in relation to gender, women represented the highest rate of alloimmunization, as they have other forms of exposure besides transfusion.