Your body contains certain blood cells, called white blood cells (WBCs), which fight infections. One subset of these WBCs is called lymphocytes, which can be divided into two main teams: T cells and B cells. The T cells and B cells can be further subdivided into categories, such as helper T cells or memory B cells.
In addition, your body has another subset of white blood cells called phagocytes, which specialize in search-and-destroy missions. These phagocytes find and then “eat” foreign particles such as bacteria, viruses, and fungi. They also clean up after dead or injured cells.
When your body is exposed to a new pathogen, say the chickenpox virus, the phagocytes are the first line of defense. Some of the phagocytes, called granulocytes, continue to eat as much virus as they can until they die. Other phagocytes, the macrophages and dendritic cells, also ingest the virus, but their goal is to take a chickenpox virus protein back to the immune system to raise the alarm. They travel to the nearest lymph node and present this viral protein to lots and lots of B cells and helper T cells. These phagocytes are looking for particular cells that contain the appropriate receptor that matches the chickenpox virus protein. When they find the match, it activates that special helper T cell, which then initiates the rest of the immune cascade.
But how is it possible for your body to have a certain helper T cell to match any given protein from all the possible microorganisms in the world? To oversimplify a complicated process, the body is able to create different receptors by combining the basic receptor building blocks in various patterns. Imagine having dozens of different size and different color Lego blocks. If you were fast enough and had enough hands, you could create millions of different shapes out of those blocks. If you had enough patterns, then you could likely find a match for the shape and color of any foreign block that wandered into your neighborhood.
It seems unbelievable, but the immune system has found a way to create millions of helper T cell receptors so that it can match practically any foreign microorganism that wanders into the body. The immune system does the same thing for B cells, which will become important in a few moments. This flexibility is the beauty of the immune system.
Now back to the fight against the chickenpox virus. When the helper T cell is activated, it makes multiple copies of itself, including some specialized cells called memory T cells. The helper T cells also produce activating proteins that are able to activate killer T cells. The killer T cells begin to patrol the body, searching for any cells infected by the virus. The infected cells are marked because when the virus entered a given cell, it left a protein on the outside of the cell wall. If the killer T cell sees that protein, it destroys the cell before it can release any more viruses.
If you go back a few steps, you will notice that the phagocytes presented the chickenpox virus protein to both helper T cells and B cells. When the virus protein matches a particular B cell, it partially activates that B cell. However, if the activating T cell protein comes into contact with a partially activated B cell, the B cell becomes fully activated. The activated B cell then starts to divide into two sets of cells: plasma cells and memory B cells.
Plasma cells are specialized to produce antibodies, which are small proteins that match the receptor on the surface of the original B cell. Remember that the chickenpox virus protein matched the receptor on the B cell that turned into this particular plasma cell. This means that the antibodies produced by the plasma cell will also match the chickenpox virus protein. Plasma cells can produce thousands of antibodies per second. These antibodies swarm all over the body and attach themselves to any chickenpox virus they find.
When the virus is coated with antibodies, it becomes an easy target for the phagocytic cells. These cells either eat and destroy the virus or present the chickenpox virus protein to the B cells and helper T cells, starting the whole cycle over again.
So what about the memory B cells and the memory T cells? These cells are programmed to have a prolonged life. Their presence allows the body to react more quickly to a repeat exposure to the chickenpox virus.
The first time your body sees the chickenpox virus, it takes over two weeks to create all the necessary antibodies to fight off the infection. However, if you have already had the infection, the memory B and T cells already exist in your body, and you are able to mass produce antibodies much more quickly. Within just a few days, there are billions of antibodies available to attack the virus, and you don’t get sick. This is why it is rare to ever get chickenpox twice.
But even though it is rare to get chickenpox twice, it is not impossible. About 1 percent of people diagnosed with chickenpox will get the disease a second time. This occurs because it is possible for your immunity to wane. It might be that your body didn’t make many of the memory cells in the first place. Or it might be that you lost those memory cells over time. Whatever the reason, this just goes to show that the body is not perfect.