American English only please
The immune system of each human being goes on as a specific individual process from the immunonaissance till immunosenescence since it depends on the heredity, body’s fetal life, environment after birth, nutrition, constituting internal microbiota, sex, age, seasonality, pregnancy (in women), and many other factors.
To explain differences in organs, cells and molecules of the immune system at the individual level, there have been studying thousands of healthy volunteers, and patients of various types of pathology.
In total, genetic predisposition is found to have the largest influence on lifespan of the immune system, and its strength that may be genetically determined as currently estimated from 25 to 75 % among all factors. Quantitative certain loci mapping has identified the association with an increase in susceptibility to infections and immune-mediated diseases. Interestingly, variations in T cell counts are more strongly dependent on genetic factors, whereas B cell counts, are more environmentally affected, especially in babies and children. Children up to from 4 to 6 years do not have yet a number of mature B cells, which could produce antibodies at the high level required for the defense against reactivated encapsulated bacteria.
Age dependent involution of the thymus is the second important factor contributing to the lifespan of the immune system. Some researcher groups even think that the transplantation of new thymus is must be taken into account. The point is that the thymic involution leads to a progressive decreased output of naive T cells ready to the adaptive responses to newly invaded antigens and correspondingly protection from them. Consequently, the former might of the immune system is diminishing with age as if there would be HIV infection, but not too fatally.
We may compare the immune system to the purgatory, where not all souls have been purifying to go to heaven. The immune system is reliable and faithful in the protection of the body fighting invaders and tumors. However, in cases of progressing cancer, severe auto-immune, and allergic disorders the same immune system can betray the body in which the system lives. Unfortunately, a variety of problems of the body cannot be solved by the immune system, and immune system is designed not for the protection only. Protein molecules important for the immune system are synthesized due to the transcription of both “good” and “bad” genes, and consequently immune system may execute both beneficial and harmful roles in the body. That is the dirty little secret of the immune system. Being a tool of the hereditary program embedded in the individual genome, immune system has been triggering at the appointed hour the senescence processes to lead body to death. Such is a law of the natural world.
However, while we are alive we should look after our immune system to prolong its individual life, and the lifespan of ourselves.
Reader: I did not see, until after I recorded the text, that there was a suggestion to "make any changes in the text".
Does this mean that the person posting would like minor errors in English to be corrected?
I did not do so in this case. Very minor errors I noticed in the text did not affect intelligibility. I could edit those in the future, however.
Good luck with your project.