M protein

noun

ˈem- 

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