X referring to any thing…. Even more specifically why can’t immunity be built against HIV/AIDS (or any other serious illness/disease)?
In: Biology
The question is a bit too broad to really answer. It can mean a lot of things.
in the context of allergy, early exposure in life to a variety of allergens has been shown to promote the regulatory side of the immune system (i.e., your body becomes better at recognizing things that are not dangerous as not dangerous). For further information, an important clinical trial on this was LEAP 2012 and a related hypothesis is called the “hygiene hypothesis”.
In the context of certain infectious diseases, there are many circulating pathogens in the human population. Various species of rhinovirus cause colds. Children catch 3-6 of these every year, but by adulthood are immune to most of the circulating ones in the population (particularly the more pathogenic type C kind). Because many of these viruses are so similar, immunity to some can give some protection to many. Adults therefore tend to be generally more resistant to colds and highly immune to the commonly more pathogenic “childhood” colds.
In the context of vaccines, exposure to what are called antigenic portions of a disease (either from a weakened living version of the disease or a non-living protein portion of the pathogen) trains (usually) a B cell response which produces specific antibodies to those proteins. When you are infected by the actual pathogen, these antibodies recognize those same proteins and trigger an immune response.
As for HIV, humans actually produce a very robust immune response to HIV infection. And the immune system is also very good at suppressing the virus… for a time. But HIV infects a portion of the immune system and it incorporates its genome into those cells. The immune system ends up killing itself by targeting these infected cells, eventually leading to a depletion of the immune system (in lay terms, you can think of active HIV infection as a rapid aging of the immune system). Some HIV therapies not only specifically target the virus, but actually (somewhat counterintuitively) work to inhibit the immune system to prevent this ongoing destruction.
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