Much of our current understanding of IBM comes from the study of IBM muscle pathology, which typically shows both immunological and degenerative processes.
Immunology: IBM muscle viewed under the microscope usually contains inflammatory cells, particularly T-cells. Cells are often congregated into nodular collections.
Although the invasion of muscle cells by T cells is frequently emphasized, most T-cells surround or displace myofibers (muscle cells) with very few invading them.
It is likely that T-cells are injuring myofibers by their secreted products.
Microarray studies published in 2002 showed an abundance of immunoglobulin transcripts in IBM muscle and led to the recognition in 2005 that plasma cells (CD138+ differentiated B cells) are transcriptionally active, producing and secreting immunoglobulins within IBM muscle, and in 2007 that these plasma cells are antigen-directed and clonally expanded. An autoantibody marker of IBM was discovered in 2011.
Nuclear degeneration: Rimmed vacuoles and the redistribution of nuclear nucleic acid-binding proteins indicate some form of nuclear degeneration is occurring in IBM.