Our immune health is inextricably tied to our overall health. The ability to fight off a viral infection, for example, is not just a matter of having acquired serum antibodies to the viral pathogen.
Our defense begins with the innate immune system, which includes all outer and inner surfaces of our body (the skin and mucous membranes), and also scavenger white blood cells (aka phagocytes -- monocytes, neutrophils, dendritic cells, macrophages), signalling proteins (cytokines), and Natural Killer cells.
The innate immune system is fast acting and responds within minutes or hours to an infection or injury. It responds in largely the same way, whatever the pathogen is, and has a relatively limited ability, but an ability nonetheless, to become trained by an exposure to a pathogen, so that it can to some extent 'remember' how to respond to the pathogen the next time that it encounters it.
If the innate immune system can't take care of the infection, then the adaptive, or acquired, immune system kicks in.
The adaptive immune system can hone in more precisely on pathogens and can 'recall' in finer detail how to respond to a pathogen the next time it encounters it, via the production of acquired antibodies and memory-B and memory-T cells. In the germinal centers of lymph nodes, acquired antibodies are tested against the invading pathogen, and only the ones with the highest binding affinity to epitopes (binding sites) on the pathogen are selected for replication. But this training takes time, which is why the adaptive immune response to the first encounter of a pathogen takes days or weeks, rather than minutes or hours.
A robust innate immune response is the reason why most children are able to shrug off an infection by SARS-CoV-2 with mild, or no, symptoms. Children haven't yet lived long enough for their innate immune system to have been degraded by a decline in overall health.
In particular, children have more innate, or natural, antibodies than adults do. These are antibodies that are already present in tissues throughout the body, and do not require training by first exposure to a pathogen. Natural antibodies can't bind as strongly to a specific pathogen as acquired antibodies to that pathogen do, because they haven't gone through the training against that pathogen that acquired antibodies undergo. But they can bind to a broader range of pathogens than acquired antibodies do, precisely because of their lack of training against a specific pathogen. And innate antibodies can react almost immediately upon the first exposure to a pathogen, whereas acquired antibodies need to undergo their training against it over days and weeks.
Is there anything that adults can do to make their immune system more robust and youthful?
The first thing is to improve our overall health, especially by correcting any insulin resistance that we may have. Insulin resistance is the scourge of modernity and underlies the chronic diseases of modernity. We can do this by exercising regularly; eating more fresh vegetables, fruits and healthy fats, including saturated fat; limiting consumption of refined carbohydrates; getting enough sleep; and thereby maintaining a healthy BMI.
There are also nutritional supplements that can boost our immune response and keep it in balance, so that inflammation, our body's initial response to injury or infection, does not get out of control. The following are some of them: