The immune system. The vaguest claim on the shelf
"Strengthens the defences" appears on products with nothing in common. What sits behind it, which ingredient can be calculated at all, and why we treat this category soberly on purpose.
No claim is as popular and as undefined as this one. "For a strong immune system" appears on gut powders, on oils, on vitamin pastes and on treats, and those products have little in common.
Why the category is so blurred
The immune system is not an organ with a dial. There is no measure in a healthy animal that a product could improve, and therefore no trial showing that a healthy dog becomes "stronger" with a powder than without. What exists are trials with concrete endpoints: how often diarrhoea occurred, how long it lasted, how a blood value changed.
So we treat the category soberly: we show what is in a product and in what amount, and rate only what has a reference with a source behind it.
The ingredients, and what survives
Probiotics are the one ingredient here with an amount we measure against. They are dosed per animal rather than per kilogram, and their established endpoints are in the gut, not in some general defence. In cats the benefit is better supported than in dogs, and the ingredient page says where that comes from.
Zinc and vitamin E take part in immune function; that is not in dispute. We still do not rate their amounts: the requirement is defined per 100 g of dietary dry matter rather than per kilogram of animal, and a complete food already covers it. More is therefore not an improvement, only more.
Taurine appears in this category because it is essential for cats. What matters, though, is not a healthy animal on a complete food but a raw or home-cooked ration, where it can be missing.
What actually helps, and does not come in a tub
The routine care plan your vet sets out with you, a ration that meets requirements, a sensible weight, exercise, sleep, and for outdoor cats the usual routines. That sounds unspectacular, and it is the part where the evidence is unambiguous.
If you want to add something beyond that, buy a product for one concrete concern whose amount can be checked, rather than one promising everything at once. Supplements are not medicines and cannot treat any condition.