The sacroiliac joints (SIJs) are one of the most common sites involved in axial spondyloarthritis (axSpA), and there are few studies on the histopathology of the SIJ in this group of patients. Mononuclear cell infiltrates in the bone marrow and fibrous tissue resembling a pannus formation were the pathological features of early sacroiliitis in our previous study. We undertook a further immunohistological evaluation of these features in patients with axSpA. Biopsy specimens from the SIJ of 6 patients with established ankylosing spondylitis (AS) and 13 patients with nonradiographic axial spondyloarthritis (nr-axSpA) were analyzed. An immunohistological method was performed to examine the macrophages (CD163), T cells (CD3), and B cells (CD20). Mononuclear cell infiltrates in the bone marrow were observed in only 6 patients with nr-axSpA. Fibrous tissue was observed in all patients with established AS and 9 patients with nr-axSpA. Macrophage, T cell, and B cell infiltrates could be detected in both the bone marrow and fibrous tissue. All bone marrow specimens from 6 nr-axSpA patients exhibited CD163+ macrophage infiltrates; of these, 5 exhibited CD20+ B cell infiltrates and 3 exhibited CD3+ T cell infiltrates. Among the fibrous tissue specimens, all exhibited macrophage infiltrates, 9 exhibited B cell infiltrates, and 4 exhibited T cell infiltrates. In addition to macrophages and T cells, B cells are also involved in active sacroiliitis in patients with axSpA.