In a more indirect fashion, the health care industry directly benefits from its largest customer base: senior citizens, and the income they receive from Social Security. Medicare is a good, pretty inclusive program, but it doesn't cover everything, and hospitals, doctors, therapists and drug companies all benefit from the extra dollars that end up in their bank accounts because of Social Security.
The interest group (I think that the term "special interest group" is a pejorative one since it implies that these groups are asking for special and undeserved favors) that is most interested in Social Security is senior citizens and those who represent them. In terms of an organization, the best known is the AARP.
The AARP is interested in lobbying for things that they think are beneficial to senior citizens. Social Security is one of these things because it represents a large portion of the incomes of some seniors. It also represents America's committment to making sure that the elderly have a basic standard of living after they retire.
There are no major groups pushing to reform Social Security. This is one reason the program is in such trouble. There is no major constituency whose main desire is to reduce the costs of Social Security. Meanwhile, maintaining Social Security (and Medicare) is the main desire of the AARP.
There are stricter criteria for debt collectors and loan companies to garnish Social Security and other types of federal benefits. Banks are prohibited from garnishing federal benefits unless the account in question has a 2 month supply built up, and people who depend on such benefits seldom are able to achieve that. However, federal benefits can nevertheless be garnished by parties other than banks. I found this here: New rule makes Social Security benefits off-limits for garnishing