In medical language, one speaks of acquired immunity to indicate a highly specific immune response, which develops in response to a given infection.
The first time our immune system faces a new infection, it is in some ways unprepared; it can in fact count on a well-stocked army, but it still knows little about the opponent's military strategies. Often, for example, pathogens are very skilled in camouflaging themselves and penetrating the immune defenses, avoiding controls. For this reason, the first contact with the pathogen produces a slow and quantitatively unimportant reaction.
Fortunately, the immune system retains a memory of the antigens it came into contact with. This memory is entrusted to special cells, called memory cells; after the first infection, these cells enter a state of quiescence, ready to intervene in case the same antigen returns. If and when this occurs, the immune response is therefore much more rapid, effective and prolonged.
For some diseases, acquired immunity protects against further infections for life or for many years. This is the case, for example, of smallpox, measles, mumps, pertussis, poliomyelitis, rubella and tetanus; not by chance, the vaccination takes advantage of this very principle: by introducing into the body a small amount of antigen (an inactivated or killed microorganism, or some of its constituents), the vaccine induces the production of memory cells. These cells will allow the body to react effectively to any further contact with the infectious agent.
Unfortunately, some viruses - such as colds, AIDS and flu - are subject to a high rate of mutations . If on the one hand this complicates the development of a vaccine enormously, on the other it causes the diseases to recur in the same individual over time. Because of this, we get sick so often with colds, especially in childhood up to adulthood.
