Direct and indirect lymphocyte participation in the immunity and immunopathology of tropical canine pancytopenia—A review

Abstract Erlichia canis , a rickettsial pathogen which infects monocytes, induces generalized lymphocyte activation. Activated T lymphocytes differentiate into effector cells capable of destroying infected and uninfected monocytes and platelets. Activated B lymphocytes differentiate into plasma cells associated with plasmocytosis, hypergammaglobulinemia with high levels of specific antibody and a platelet migration inhibition factor. These effector mechanisms, aimed at parasite destruction, contribute to the pathogenesis of acute tropical canine pancytopenia and do not completely eliminate the etiologic agent. A carrier state of ‘infection-immunity’ exists between the immunocompetent host and virulent parasite. E. canis has evolved mechanisms of ‘tolerant-symbiosis’ with hostile macrophages and other effector cells in the immune host.