MUSK and myasthenia gravis: In myasthenia gravis (MG), B cells are primarily involved in antibody production against the AChR (primarily IgG1 subclass antibodies) and muscle-specific kinase (MuSK, primarily IgG4 subclass antibodies) [192, 193] but potentially also in antigen presentation given their BCR specificity and T cell involvement [194–197].