How do lymphocytes (the cells that recognize and kill infected cells) notice, that a cell is infected?
Cells in your body have specific protein complexes on their surface which provide other cells with information, receive information or do other useful stuff.
One of these receptors is called “MHC” (major histocompatibility complex). Using MHC, the cell constantly presents a sample of the proteins that are currently being synthesized on the inside. The lymphocytes check those proteins (compare them to known antigens) and usually classify the body’s own proteins as harmless. 
When a virus infects a cell, it causes the cell to produce the proteins the virus needs for its replication cycle, sometimes even completely stopping the cells own protein synthesis. The MHC complex thus presents virus protein to the lymphocytes, which recognize the protein as dangerous and swiftly kill the cell.
This way, the virus is prevented from multiplying as it requires a host cell for this process.
There are some exceptions to this mechanism. Red blood cells (erythrocytes) don’t have a MHC complex. A disease like malaria (a parasitic infection, not viral, there are still foreign proteins being produced) can thus stay undetected by the lymphocytes as long as the parasite stays inside the cell.
Some viruses, like the herpes virus, can “hide” by not forcing the cell to produce virus proteins. The herpes virus embeds itself into cells of the nervous system and only induces protein synthesis when the time is right. This way, a herpes infection can go unnoticed for a very long time before it suddenly breaks out (because of an organ transplant and the resulting immunosuppression for example).
HIV is even “smarter” because it attacks the lymphocytes themselves. The cells don’t realize they’ve been infected and the virus happily proliferates, killing the lymphocytes and ruining the adaptive immune system.
The result of this is AIDS, which allows other pathogens that have been previously controlled by the immune system to spread through and kill the body.
But more about that one another time.
Lecture “Herpesviruses” by Prof. Friedrich Grässer
Lecture “Components of the innate and adaptive immune system” by Barbara Walch-Rückheim
First picture taken from pixabay.com, scribbles by me
Got a scientific topic which you want to see as a story? Leave me a comment!
You want to support scientists on Steemit? You are a scientist on Steemit? Join the #steemSTEM channel on steemit.chat and connect with us!
STEM is an acronym for Science, Technology, Engineering and Math
@saywha
@atopy