How are T-Cells Activated?!?
Some background info: What is the immune system?
Our immune system is a complex and highly adaptable system in our body. The role of our immune system is to protect us from the billions of bacteria and viruses we interact with everyday.
Imagine yourself simply going out to get some groceries. Well guess what…every square inch of the handle of your shopping cart has almost 8,000 colonies of bacteria per square inch! Good thing they usually have wipes.
Well the reason you don’t get sick everytime you go shopping is thanks to your immune system.
The immune system has many different types of cells, divided into the innate and adaptive immunity. Innate immunity refers to the first line of defence your body does against viruses. This could be your skin or even the mucus in your nose. But the adaptive immunity however is much more advanced. Being “adaptive” refers to its ability to learn and make targeted attacks.
When looking at our adaptive immunity we have two main cells.
- B Lymphocytes
- T Lymphocytes
There is also a third type of lymphocyte that was discovered that has properties of both the B and T lymphocytes call an X Lymphocyte, but it is still very mysterious and we still dont know its exact function.
B cells/ B lymphocytes- The main function of B-cells is to recognize antigens. It is also responsible for producing antibodies. A crucial weapon for the immune system in fighting of invaders.
Antigen- anything the body recognizes as a forgin substance.
- Bacteria
- Virus
- Fungus
- Chemical
T Cells/T Lymphocytes-responsible for activating an immune response.
T cells are almost like the generals of our immune system. They have the responsibility to give orders and tell other cells what they need to do. All the way from telling somatic cells(body cells) to cause apoptosis (cell suicide) or telling other immune cells to start producing antibodies. Antibodies are small little “handles” that can attach to the antigens receptors and “kill” it.
But how are these T-Cell activated…
We certainly don’t want them turning off and on as they wish. They could cause all sorts of unwanted battles in our body.
All T-Cells are inactive at the beginning.
Well… they are three main steps for T-Cell activation. This is where a Naive T-Cell gets activated into an Effector T-Cell/ Mature T-Cell.
First of all T-Cells need an APC (Antigen Presenting Cell) to get activated. Some examples of APC’s are macrophages, dendritic cells, langerhans cells, and B- lymphocytes. All APC cells atleast one of the two cell surface receptors.
- MHC (Major Histocompatibility Complex) 1- This specific type of receptor is only on Nucleated Cells. This also limits them to be able to interact with only CD8+ Cells. CD8+ is a specific type of T-Cell a.k.a killer T-Cell.
- MHC (Major Histocompatibility Complex)2- This specific type of receptor is only on Un-nucleated Cells. This also limits them to be able to interact with only CD4+ Cells. CD4+ is a specific type of T-Cell a.k.a helper T-Cell.
Now to how T-cells are activated…
- Antigen-specific or commonly refered to as Stimulation- This is the first “alert” that the naive T-cell will recieve. This is an interaction between the MHC surface receptor on the APC cell and the TCR receptor on the naive T-cell.
- Co-stimulation of Second Signal- Co-stimulatory molecules are a heterogenous group of cell surface proteins that amplify activating signals provided to T cells from the T cell receptor (TCR) following its interaction with an antigen/major histocompatibility complex (MHC). This leads to T cell differentiation. This secoundary stimulation is also important because it stops the cell for going into anergy. Anergy is the absebce of an immune reponsce. Molecules like CD80 and CD86 are examples of co-stimulation molecules on the APC. These molecules most commonly will interact with the CD28 maker on the T-Cell.
3. Potent Signal for Production of Interleukins- This third and final step is the production of interleukins. This is a class of cytokines that are expressed by white blood cells. During this step we will also see the T-Cell differenciate into CD8+ or CD4+.
Now finally that we have our mature/ effector T-cell, we can move on! This T-Cell then has the potential to give instructions to the B-Cell about antibody production or go ahead right into the field of battle.
