Background: Epidemiologic studies show the high prevalence of some infections and cancers in individual blood groups. Perhaps, level of immunity factors differs in people with individual blood group. The aim of this study is to compare the level of antibodies and complement factors in ABO blood groups.

Materials and Methods: In this descriptive study, peripheral blood samples from 40 male healthy individuals with different ABO blood groups (n=10 in each group) with similar age (18-25 years) and genetic background were collected. The serum levels of total IgG, IgM, IgA, C3, and C4 were analyzed by nephelometry method. All data were analyzed by SPSS software version 11.5.

Results: The levels of IgA, IgG, and C4 protein in individuals with O blood group were significantly higher than those of other groups (p=0.02, p=0.01, and p=0.004, respectively). Also, the mean concentration of IgM in individuals with AB blood group was significantly higher than that in other groups (p=0.02). There was a significant difference between the level of C3 in O blood group and those of other blood groups (p=0.01). The mean concentrations of all parameters (except IgG) in B blood group were lower than those in other blood group.

Conclusion: Generally, natural antibodies in O blood group were higher than those in other groups. This high level of total antibodies in O blood group may reduce susceptibility to some infections.