Mary Douglas's "Purity and Danger" offers a structuralist take on how cleanness and uncleanness serve as symbolic functions to maintain society's boundaries. The argument, following Emile Durkheim's notions about the relation between clean and unclean, is (very briefly) as follows: being "unclean" is being out of place. The notion of profanity or defilement is in essence the notion of transgressing social boundaries (this can be seen, for example, in how taboos relate to impurity). The culturally and historically dependant dichotomy between pure and impure, clean and unclean, is to a great extent a means of reinforcing norms, norms being themselves agents of establishing social structures and hierarchies.
If we take Douglas's (revised in 2002) discussion on Jewish Kosher rules as an example, we can hypothesize a social situation in which pig is the cheapest meat customarily consumed by lower and poorer classes. The ruling classes, aiming at reinforcing their privileged social position, maintain pig to be profane, thus fashioning their abstention from pig into an instrument of increasing their symbolic fortune. In other words, the pig refraining ruling classes set society's structure and boundaries, defined by the distinction between pure and impure, to fit their own habitus and make it central to society while others are marginalized.
In "Purity and Danger" Douglas argued that secular defilement is not that much different from "primitive" ritualistic practices relating to cleaning or "purifying". The social function of defilement can also be seen in modern secular societies. Take for example the ongoing delegitimizing of smoking or junk-food. Obesity is a health hazard, not a personal one but rather a social one, for obesity is associated with being poor. Being poor also makes you unaware and uniformed and the combination of both leads the way to McDonald's or KFC. The upper classes, on the other hand, make sure to eat healthy and refrain from junk food (like the Jews and Muslims do with pig). Thus a new mini-religion is founded on the constructs of social hierarchy, the religion of "healthiness" that decrees a set of codes and regulations about what you can and cannot eat. These new boundaries between deep-fried bad and organic good reinforce the existing social structure and hierarchy.
See also: Mary Douglas - Ritual Uncleanness
Purity and Danger by Mary Douglas: extended summary