BACKGROUND: Fetal‐neonatal alloimmune thrombocytopenia (FNAIT) diagnosis relies on maternofetal incompatibility and alloantibody identification. Genotyping for rare platelet (PLT) polymorphisms allowed the identification of three families with suspected or confirmed maternofetal incompatibility for the αIIb‐c.2614C>A mutation (Halle et al., Transfusion 2008;48:14‐15).STUDY DESIGN AND METHODS: A polymerase chain reaction–sequence‐specific primers amplification assay was designed to genotype the αIIb‐c.2614C>A mutation. HEK293 cells expressing αIIb‐Leu841 or αIIb‐Met841 αIIbβ3 forms were used to probe the reactivity of maternal sera from these families and to study the effects of the substitution on αIIbβ3 expression and functions.RESULTS: Tested by flow cytometry (FCM), one serum sample specifically reacted with αIIb‐Met841 but not with αIIb‐Leu841 αIIbβ3. This specificity revealed the αIIb‐Leu841 polymorphism as a new alloantigen named Cab3a+. Cross‐match testing using FCM also showed the Cab3a+ antigen to be expressed at the PLT surface. As for anti‐human PLT alloantigen (HPA)‐3a (or ‐3b) and anti‐HPA‐9bw, detection of anti‐Cab3a+ alloantibodies appeared difficult and required whole PLT assays when classical monoclonal antibody–specific immobilization of PLT antigen test failed. In our FNAIT set, the immune response to Cab3a+ maternofetal incompatibility could induce severe thrombocytopenias and life‐threatening hemorrhages. The p.Leu841Met substitution has limited effects, if any, on local αIIb structure, preserving both αIIbβ3 expression and functions.CONCLUSION: The Cab3a+ polymorphism is a new rare alloantigen (allelic frequency <1%) carried by αIIb that might result in severe life‐threatening thrombocytopenias. In Sub‐Saharan African populations, higher Cab3a+ gene frequencies (up to 8.2%; Halle et al., Transfusion 2008;48:14‐15) and homozygous people are observed.