Medical Definition of M protein
1 : an antigenic protein of Group A streptococci that is found on the cell wall extending into the surrounding capsule, is variable among the different serotypes, and confers streptococcal virulence by protecting the cell from phagocytotic action
— called also M substance
2 : a monoclonal antibody that is produced by plasma cells abnormally proliferating from a single clone and that is characteristic of monoclonal gammopathy
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