Anti-M antibodies: Biphasic (reactive at room temperature and at 37°C): A case series.

Anti-M antibody, which is not reactive at 37°C, is not clinically significant. Reports of clinically significant anti-M antibodies causing hemolytic disease of the fetus and the newborn (HDFN) and delayed hemolytic transfusion reaction (DHTR) are available. We report 13 cases of anti-M antibodies reactive at room temperature (RT) and at 37°C. These were found in patients of varied age groups (11 months to 85 years) with varied clinical diagnosis. All the female patients were multigravida. In all cases, antibody screening was positive at RT as well as at the indirect antiglobulin test (IAT) phase. Providing "M"-antigen negative transfusions is the best therapy in this situation. Provision of red blood cell (RBC) antigen phenotyped donor registry shall ensure quick provision of antigen-negative blood for transfusion in emergency situations.

Introduction

The antibodies are considered to be clinically significant when they are reactive at 37°C and are frequently associated with hemolytic disease of the fetus and the newborn (HDFN), hemolytic transfusion reactions (HTRs), or a notable decrease in the survival of transfused red cells.[1] Antibodies from the MNS, P, and Lewis blood group systems are considered to be clinically insignificant as they usually appear as cold-reactive antibodies.[1] Some examples of anti-M antibody, however, are known to be clinically significant in nature as they are reactive at 37°C and have been known to cause HDFN[23] and HTRs.[45] MNS antigens are expressed even on cord blood cells.[6] We are reporting 13 cases of anti-M antibody identified in our patients that were found to be reactive at room temperature (RT) and at 37°C (the antiglobulin phase).

Case Report

We had carried out an unexpected red cell antibody screening in 9,546 patients (randomly selected) admitted in our institute for transfusions or surgeries for a period of 2 years (March 2011-March 2013). Ninety-three patients had developed alloantibodies. Thirteen out of these 93 patients (13.98%) were identified with anti-M antibody and were from different clinical specialties [Table 1]. Red cell antibody screening test was carried out using three cell screening panel cells by column agglutination technology (CAT) (ID-DiaCell I-II-III, Bio-Rad Laboratories, Cressier, Switzerland). When the antibody screening test was found positive, antibody identification was carried out using 11 cell identification panel cells (ID-DiaPanel, Bio-Rad Laboratories, Cressier, Switzerland) and 11 cell enzyme treated (papainized) panel cells by CAT (ID-DiaPanel-P, Bio-Rad Laboratories, Cressier, Switzerland). The anti-M antibody was reactive at both the phases of testing, that is, at RT and at the anti-human globulin (AHG) phase, which is usually not observed. On further treatment with enzyme treated panel cells, the reactions were found to be negative. Enzymes, such as papain, cleave the red cell membrane sialoglycoproteins of the MNS blood group antigens at well-defined sites. The reactivity of anti-M antibody is abolished and thus, sensitivity of the M antigen to the proteases helps in the identification of the antibody.[1] Autologous control test was carried out using CAT at both RT and AHG phase. DAT was carried out using CAT. In all these cases, autologous control and DAT test results were found to be negative.

Table 1

Clinical presentation of patients

The patients' age ranged widely from 11 months to 85 years with a mean of 46.37 years. Out of 13 patients, 10 were male patients and 3 were female patients. Two out of the male patients had history of prior transfusion and all the female patients were multigravida. The hemoglobin level of the 13 patients in whom anti-M was detected ranged 7.1-11.9 gm%. Out of these 13 patients, 10 required transfusion, and hemoglobin level in these patients was ≥8.2 gm%. It was observed that average increase in the hemoglobin level was 1.2 gm% per unit of red cell transfused in these patients. The M-antigen status of these 13 patients was confirmed to be M-antigen negative using anti-M antisera and N-antigen status was not determined.

One hundred and ninety-two randomly selected donor blood units were screened for the M-antigen status using anti-M antisera, of which 42 (21.87%) units were found to be M-antigen negative. These units when cross-matched were found to be compatible in the indirect antiglobulin test (IAT) phase. Thirty-one units were transfused to ten patients requiring transfusion. Out of these ten patients, four patients were available for the follow-up and hemoglobin was maintained above 11 gm% in these patients. No evidence of DHTR was seen in any of these patients and no new alloantibodies developed in them in a follow-up period of 6 months. In one of the four patients, anti-M antibody was not detectable after 3 months.

Discussion

The most commonly encountered antibodies from the MNS blood group are directed against the M, N, S, antigens. Anti-M antibody most often occurs as a naturally occurring saline agglutinin. It is predominantly of the immunoglobulin M (IgM) type, but few may be found as partly or wholly of immunoglobulin G (IgG) type. It is also observed that anti-M antibody is found in sera of patients who had no exposure to red cells. Thus, anti-M antibody is not considered to be clinically significant but when found reactive at 37°C or at AHG phase, it should be considered clinically significant. It is known that antigens of the MNS blood group system are sensitive to treatment with enzymes, such as papain and ficin, as these enzymes cleave the red cell membrane sialoglycoproteins at well-defined sites. The reactivity with anti-M antibody is abolished and thus, sensitivity of the M-antigen to the proteases helps in the identification of the antibody.[1]

Frequency of anti-M antibody in our study population was found to be 13.98% (13/93). Petras et al., in their study, reported the frequency of anti-M antibody as 2.9% (197/6769)[7] and Tormey et al. as 3.45% (18/521).[8]

M-antigen status was determined in our donors to transfuse blood to the recipients. M-antigen frequency was found to be 78% (150/192) in our donor population. The frequency of M-antigen varies with population. In Caucasians, the frequency of M-antigen has been reported to be 78%, in blacks it is 74%,[6] in Europeans and African Americans the frequency was reported as 78% and 70%, respectively.[9] Thakral et al., in their study from North India report the frequency of M-antigen to be 75.39%.[9] Makroo et al. reported the prevalence of M-antigen as 88.7%.[10] However, there, have been no reports in the literature from the Western India.

In our study, all the 13 cases of anti-M antibody identified were biphasic in nature. Since they are reactive at 37°C, it may be considered as clinically significant and further be investigated for the potential to cause HTR and HDFN. However, since we have provided “M”-antigen negative blood to these patients, the possibility of these antibodies causing HTR could not be established. We successfully transfused our patients with M-antigen negative IAT compatible units; approximately 15-20 units were randomly cross-matched per patient to obtain one IAT compatible unit. We have not done any antigen frequency studies and donors were selected randomly. Probability of finding “M”-antigen negative donor is one in five in our experience. Provision of red blood cell (RBC) antigen phenotyped donor registry shall ensure quick provision of antigen negative blood for transfusion in emergency situations.

Financial support and sponsorship

National Health and Education Society (NHES).

Conflicts of interest

There are no conflicts of interest.