Is airlines food safe for kidney transplant?

At high altitude, all foods have less flavour. (I appreciate that sounds a bit odd, but trust me on this - Heston Blumanthal analysed the effects of altitude on perception of taste). To make up for the lack of flavour, more fat and salt is required in the dish, to make it more palatable. So if you would not normally eat a potentially heavily-processed ready meal even at regular altitude, it's probably best to avoid airplane food. Or just take your own.

Also, my own personal experience as an immunosuppressed transplant recipient has taught me never to eat prepacked sandwiches. For me, they have a couple of minor episodes of food-poisoning (local canteen sandwiches) and one intensely horrible episode of food-poisoning (tesco sandwich). These days I figure it's safer not to go near them. (I think part of the problem is that supermarkets tend to use "offcuts" in sandwiches, and it's not possible to guarantee storage conditions are cold enough for them to remain completely safe).