What are the factors that strengthen or weaken a person’s immunity? Why are some people better able to fight things like infections and viruses than other people? And if a person knows he’s going to be in a situation that will challenge his immunity, like major surgery, what can he do ahead of time to increase his resistance?
You may not like my answer - I would say that there’s little anyone can do to “boost” his/her immune system notwithstanding the many claims made to the contrary.
Recognized suppressant factors include diseases like AIDS (of course), cancer (especially of the immune system cells, i.e. leukemia and lymphoma), old age, diabetes, severe malnutrition, kidney disease, and medications such as steroids (e.g. prednisone and other glucocorticoids, not the “steroids” talked about with respect to muscle build-up) and cancer chemotherapeutics which, in fact, are also sometimes used as immunosuppressants to treat various so-called autoimmune diseases. Aside from stopping any offending medications and correcting gross malnutrition, there’s not much room to intervene regarding the above factors.
Wouldn’t it be realistic though, to make the argument that vaccination has a profound immune-strengthening effect?
Bonus question: what’s the difference between vaccination and immunization? 
There are some reasonable steps you can take that are linked, at least to some degree to lower susceptibility to infection. As summarized here, they are:
*Don’t smoke.
Eat a diet high in fruits, vegetables, and whole grains, and low in saturated fat.
Exercise regularly.
Maintain a healthy weight.
Control your blood pressure.
If you drink alcohol, drink only in moderation.
Get adequate sleep.
Take steps to avoid infection, such as washing your hands frequently and cooking meats thoroughly.
Get regular medical screening tests for people in your age group and risk category.*
I find it interesting that on the one hand, the Harvard Health Report advises us to be skeptical, particular about products marketed to “boost immunity” (good advice), but on the other hand cites a hypothetical example of an elderly lady who thinks the herbal preparation she’s taken all her life has protected her from illness, and says “who’s to say” she isn’t right? (since, like, we’re all different, you know). This is the same faulty logic alt med supporters use to justify trying all kinds of supplements and therapies in the hope that eventually you’ll find one that works for you, despite zero evidence of effectiveness in tested groups (assuming any real research has been done at all). What’s more logical is that granny has been lucky, has some genetic predisposition to be healthy, or has benefited from immunization (or been protected by the herd immunity of others who’ve gotten vaccinated).
The Harvard Health Report also promotes the idea of a daily multivitamin, which has recently come under scrutiny for not just being unnecessary for most people, but also for possibly promoting increased susceptibility to disease, including certain cancers (antioxidants are not unqualified magic bullets - see recent research regarding prostate cancer, for instance).
Diabetes is a BIG factor. And there are a lot of people walking around who are Diabetic or pre-Diabetic and either don’t know it, or won’t admit it. If you are overweight, if Diabetes runs in your family, or if heart disease runs in your family, you could be Diabetic.
Diabetes can be controlled, but never “cured.” Type 2 Diabetes can often be managed by diet and exercise, but this does not “cure” it. Bottom line, you STILL have Diabetes, and in the event of an accident, illness, or major surgery, you are going to need the Diabetes acknowledged and managed. Diabetics, even those with excellent control, take longer to heal, and are more susceptible to infection.
~VOW
Sorry, I would like to ask for cites for any/all of the factors above that show heightened immune function with a clinically meaningful endpoint such as less infectious/neoplastic illness (so I don’t mean some ex-vivo assay of neutrophil function that shows it phagocytizes better or similar “evidence”)
Bonus first - none. “Vaccination” owes it origin to the use of the cow pox virus (vaccinia, vacca being the Latin root for cow) to immunize against the related virus small pox. (1796, Edward Jenner, a great story.)
And yes, immunization has profound immune-strengthening effect. So does having had past exposures to infectious agents that you have recovered from.
Karl, will this do? Maybe combined with an animal model study? Or will you hold out only for a study that shows two groups of humans randomized into chronic sleep deprivation or adequate sleep and having different infectious disease endpoints?
Sorry, won’t do. The first study is a classic example of using a surrogate endpoint. It doesn’t tell us about the whole organism in a real, clinical setting. Who knows, maybe having those extra Th1 cells will lead to more autoimmunity! The second study is more convincing but, in addition to using (an admittedly more reasonable) surrogate endpoint, it doesn’t inform about human responses.
If I sound hung up about surrogate endpoints, it’s because they can lead you down a garden path to . . . death. An example? In the CASS trial of suppression of VPBs (PVCs), i.e. suppression of ventricular ectopy, following an MI, the VPBs were nicely suppressed by the study drugs (encainide, et al). Odd thing, though . . . mortality was increased. Attempts to fudge things with changing enrollment criteria and even by using different drugs led to the same thing: suppression of VPBs and higher mortality. I can give other examples, if you want. Bottom line, I need clinically relevant endpoints. 
Oops, I had meant to give a link to the CASS trial.
I’m curious about the first of the OP’s questions. Why do some people have stronger immune systems? I’m not referring to more immunity relative to people with diseases but rather, more immunity relative to other healthy people who have good health habits. Is it always genetic?
I have an unidentifiable auto-immune disorder. (It surprises me how many auto-immune disorders don’t fit the profile of a recognized, named disorder like lupus erythematosus.) Auto-immunity means only that my body tends to attack itself; yet I find it interesting that I very rarely get sick and haven’t had a cold in probably 15 years. I hasten to add that I am not on prednisone or other steroids that increase susceptibility to infection.
So if some people do have better immune systems than others, is it more likely that their immune systems are constantly in a state of Yellow Alert (like mine, I suspect), or that they have greater numbers of the components of immunity, such as more T-cells? Or do we have any way of knowing?
Reaching back to another career, I would have to say that immunization is the body’s response (antibodies, WBC’s) to some sort of challenge, infection or vaccine.