There are a few reasons why some infections can get the better of our immune system whereas others are more easily dealt with. First off you have some really cunning bugs which hide themselves inside cells – these can often be dealt with by CD8 T cells (because the immune system is smart too!), but if we are swamped with them in big enough numbers they can get the better of us. This is because the immune system takes a while to mount a response to a particular antigen (bad guy) and so if the bug is present in big enough numbers and is virulent enough it can gain the upper hand for a while. Most of the time the immune system will kick in and beat it in a few days but sometimes someone will die if they don’t get some kind of help from a drug that will kill the bug.
Some infections actively destroy our immune systems – like the virus causing HIV/AIDS. It is so dangerous because the infecting virus actually attacks immune cells, and so can destroy the immune system – giving people similar symptoms to those born without immune systems. This is what makes it such a devastating illness, and why it is so important to find a cure.
Another way pathogens can cause a lot of damage is by a process called antigenic mimicry. This is something that the bugs are sort of causing by mistake. Basically some of the proteins on the bacteria’s surface are very similar to those found in parts of your body. So when your immune system goes to work killing the bug it gets confused and starts attacking bits of you as well!! An example of this is the bacteria that causes Chlamydia which has a protein on its surface that is similar to something called the alpha-myosin heavy chain – which is found in your heart. An immune reaction to Chlamydia triggers your body to attack the heart muscle which causes a condition known as inflammatory heart disease – which damages the heart! Not good! 🙁
Some people are “immunocompromised” – this means that their immune system is not working to its full potential. This could be because of medication they are taking, because of another illness that they already have or because they have a problem in their DNA and were born with only a partly functioning immune system. In people with this sort of immune deficiency the likelihood of them not being able to cope with certain infections is increased a LOT!
Did this answer your question? Thanks, it was a brilliant one! 🙂