People exist in society as topics of communication. As King and Thornhill (2003) write,
For Luhmann, people . . . exist for and within social systems only in so far as these systems are able to communicate about them.. . . People do exist in the form of ‘the public,’ ‘individuals’, ‘reasonable men’, ‘rational beings’. The subjects of experiments, clients, or patients form a part of the environment for social systems, such as politics or law, science, economics, health or the mass media, but always as constructions of that system that is communicating about them.
As far as society is concerned, people in Luhmann’s scheme do not have a reality or an autonomy independent of society’s communication subsystems.
(“‘Will the real Niklas Luhmann stand up, please.’ A rely to John Mingers.” Sociological Review.)
Regarding the question of human agency, a human being–at least as understood in the commonsense way as the unity of mind and body–cannot change society because only communication can communicate; however, a social system (e.g., the mass media system) can construct an individual as an agent of social change. That is to say, an individual can be a character in a narrative or a topic of communication. In The Reality of Mass Media, Luhmann argues,
The significance of personal individualization becomes . . . clearly visible if one observes the temporal relations of narratives loaded with tension. On the one hand, the people who appear in the narrative gradually come to be known, they have names, they act, one finds out a little about their past. They are individualized through their own history. On the other hand, one still does not know how they will act, especially in situations that are as yet unfamiliar and in the face of the unknown provocations from others’ actions. In people, then, a known or at least a knowable past, at any rate one which is unchangeable, encounters an unknown future. People symbolize the unity of the known/unknown schema, interpreted through the temporal difference of past and future. (73)
The flesh and body body only enters society as a topic of communication. My brain and heart, etc., are not included in society until a social system, such as the health system, communicates about them. I can enter a function system, such as education, as a professor. A professor can be an addressee in communication or topic of communication. An unemployed or employed professor can be a topic conversation (maybe as a statistic) in economics. Statistics on employment among English professors is meaningful communication within economics–which is not to be confused with the economy as a function system. It’s not meaningful for the health system unless, for example, unemployment impacts a professor’s health.