AIG is characterised by the presence of autoantibodies against parietal cell antigens (PCA) and IF. IF is a glycoprotein which is secreted by the parietal cells. It forms complexes with vitamin B12, whose absorption in the ileum is hindered by anti-intrinsic factor antibodies (IFA). Sera from AIG or PA patients contain two types of IFA (both IgG). IFA of type 1 react with the vitamin B12 binding site of the IF, IFA of type 2 hinder the binding of the IF to the receptors in the ileum. IFA have a very high specificity for AIG. With PA, IFA occur in 40 to 80 % of patients depending on the disease stage.

Antibodies against PCA occur in patients with AIG as well as PA. They are mainly of the IgG and IgA classes. The prevalence of antibodies against PCA is very high in almost all patients with chronic-atrophic gastritis, amounting to nearly 100 %. Antibodies against PCA also have a very high sensitivity for PA, amounting to 80 to 90 % at the time of diagnosis. Over the course of disease, their prevalence decreases due to the progressing destruction of the parietal cells. With respect to the specificity of the anti-PCA antibodies, it must be taken into account that they can also be detected in patients with endocrinopathies and healthy blood donors.

For very specific determination of APCA, EUROIMMUN has developed the recombinant ATP4B as the new antigen. ATP4B stands for the extracellular domain of the β-subunit of H⁺/ K⁺-ATPase and is the main antigen of APCA. The Anti-ATP4B ELISA (IgG) thus yields a significantly higher specificity without a loss in sensitivity compared to the Anti-PCA ELISA (IgG).