I used equal signs because I thought dashes might be confusing. They haven no mathematical function in the following 'tables'

The variables in question are:

Age in years in reference to age 48

Year Waiting time in program for acceptance into the program

Surgery 0=no previous surgery; 1=previous surgery

Transplant 0=no transplant perform; 1=transplant performed

A series of Cox regression/hazard ratio tables was run, and this was the output

When run singly:

Variable = Coef = HR

Age = 0.03069 = 1.031

Year = -0.19077 = 0.826

Surgery = -0.73911 = 0.478

Transplant = 0.12567 = 1.134

When run together:

Variable = Coef = HR

Age = 0.02715 = 1.028

Year = -.014611 = 0.864

Surgery = -.063582 = 0.530

Transplant = -0.01189 = 0.988

When run together with interaction:

Variable = Coef = HR

Age = 0.02988 = 1.030

Year = -0.25211 = 0.777

Surgery = -0.66270 = 0.515

Transplant = -0.62253 = 0.537

Yr x Transplant = 0.19697 = 1.218

The question I've been asked to answer is: "Explain why transplantation is a risk for death when taken alone, is protective when used in conjunction with the other variables, and why the risk is "absorbed" by the interaction of year and transplantation. How does this cohere with the observation that survival is clearly extended by transplantation (see graph)?" (Graph shows transplantation definitively lengthens survival time).

So transplantation itself is dangerous and was more so when first being performed; surgical technique and improved immunosuppressant medications have since come into use so the change for organ rejection was higher. When the other variables are taken into account, I'd assume this risk is reduced when the other factors are considered. But I don't know why. Does anyone have any idea?

Thanks.