To I have to pick up luggage at baggage claim when changing airlines?

If your tickets are bought as a single trip, meaning they have a single PRN (6 character identifier), you might be able to check the luggage through to the destination. Otherwise, if the tickets are separate, yes you will have to retrieve the bag and recheck it meaning also another trip thru security as well.

While the luggage should be checked all the way through, as the previous commenter said, to be sure, have your traveling companion look at their airline luggage stickers If the last 3-letter airport identifier is "BOS", the luggage is only checked through to Boston. (The 3-letter identifiers should be listed on your airline itineraries.) If the last 3-letter airport identifier is the final destination, the luggage is checked all the way through.

By the way, it is unlikely that one can carry enough cloths to have clean cloths for a 4+ week trip without doing laundry somewhere along the way. Therefore, if you're going to do laundry at some point anyway, why not just pack enough to fill a carry-on sized suitcase? I can get 6 days of cloths in my carry-on...

Airlines have their own agreements. Get in touch with the airline issuing the tickets (or at least the initial airline) to see if your luggage will be transferred. Returning to the US you may have to pick up your luggage for customs inspection in Boston but if it is ticketed to your destination you quickly return it at a desk behind customs.

So you are flying US city to Boston to European city. Just for clarity, I'm going to say they're flying Pittsburgh through Boston to Dublin.

If this is all booked on one ticket, the luggage should be checked through to Dublin, and it will not be picked up in Boston. When you check in in Pittsburgh, ask, and make sure that the luggage is tagged DUB (know the code of your European airport). If for any reason it's tagged BOS, you will indeed have to pick it up in Boston and recheck it. If you are flying separate tickets, it depends on the airlines involved; sometimes they will "interline" them, meaning it can be checked through to Dublin anyway, but often these days they won't, meaning you do have to pick it up in Boston and recheck it.

On the way home, regardless of whether you are flying on one ticket or separate tickets, when you land in Boston you will go through immigration (passport control), then pick up all checked luggage, then go through customs. If you are on one ticket, your luggage will be tagged PIT; you find the "baggage recheck" desk for your airline just outside the customs door, and drop your luggage there. You then go to your new gate, passing through security. You should have been given both boarding passes when you checked in in Dublin (DUB to BOS and BOS to PIT).

If you are on separate tickets and the bags aren't interlined, on landing you go through immigration, luggage pickup, and customs as above. However, since your bags will only be tagged BOS, you then have to go to the check in counters of your next airline to re-check the bags and get your new boarding pass. You then go through security to your gate.

Harold is right, except that he picked a bad example of a European city - Dublin -- because Dublin has a US Preclearance facility. You would go through US immigration and customs in Dublin, then when you land in Boston you just leave, and go to your connecting flight, as if DUB-BOS had been a domestic flight. But with any other European city, his scenarios are correct.

Yikes, Robert is right - I indeed picked the one city where things are different! That's what I get for rushing. Just substitute London Heathrow LHR for every mention of Dublin DUB - then my post is correct.

RE US Preclearance sites.

Preclearance is in place at a number of major Canadian airports and ferry terminals. The Victoria ferry preclearance is the site that confiscated a lot of dog food. (Pretty easy to spot the dog food people). Apparently the US charges the foreign airport a fee for preclearance.

See: https://www.cbp.gov/border-security/ports-entry/operations/preclearance

...15 air Preclearance locations in 6 countries: Dublin and Shannon in
Ireland; Aruba; Freeport and Nassau in The Bahamas; Bermuda; Abu
Dhabi, United Arab Emirates; and Calgary, Toronto, Edmonton, Halifax,
Montreal, Ottawa, Vancouver, and Winnipeg in Canada. CBP also staffs a
Pre-inspection facility for passenger/vehicle ferry traffic to the
U.S. in Victoria, Canada.

Preclearance Expansion

CBP is currently negotiating with several countries interested in
establishing Preclearance operations, and recently concluded
agreements to cover Stockholm, Sweden (signed November 4, 2016) and
Punta Cana, Dominican Republic (signed December 1, 2016).

The Victoria ferry preclearance is the site that confiscated a lot of dog food.

I want to see that story. I guess maybe a Google search will turn it up.

RE Victoria ferry preclearance dog food.

Personal observation 2009 July. Departing Victoria Northwest Tandem Rally leaving Victoria by ferry for the States. The American Customs &
Boarder Protection does the passport control and customs check at the Victoria ferry site ahead of rolling onto the ferry. Spotting dogs was pretty obvious and the customs age, ts queried the dog owners about carrying dog food back into the States. A lot of dried dog food bags were handed over to the customs agents and a lot of dog vehicles made the right turn into the pet store in Port Angeles. Dogs eat meat and the US Customs prohibits bringing in meat and meat products into the States.

The Americans don't like Canadian cows and Canadian plywood NAFTA or no NAFTA.

-slaps self on top of the head-

I didn't even think that there were probably other pre-clearance sites outside of Dublin. (I really liked that when we flew home that way.) But that's good news if you're coming through one of those cities!