Here are some links that provide a good introduction to the immune system.
Here’s my version of an introduction.
The immune system is divided into 2 types: the innate and the adaptive.
- Antigens are any substance that can trigger an immune response, usually proteins.
- Cytokines are like neurotransmitters, they send chemical signals to other parts of the immune system.
- Tissue is a group of alike cells that have a function in the body.
The Innate Immune System
Cells in the innate immune system:
- Neutrophils: first responders, can release toxic granules to kill invaders
- Mast cells : involved in parasite defense (AKA allergy)
- Basophils: involved in parasite defense (allergy)
- Eosinophils: involved in parasite defense (allergy)
- Monocytes and macrophages: phagocytes (“cell eaters”) , a clean up crew that gulps up stuff and gives it to the adaptive immune system for processing
- Natural killer cells: these are actually T cells, they can kill bad cells or release cytokines.
The innate includes things like skin and stomach acid that are natural barriers to infections. Phagocytes can recognize patterns with receptors (toll-like receptors). They also respond to cytokines, which trigger the phagocytes to devour nearby stuff. Phagocytes are also called antigen presenting cells (APC’s), because the stuff they eat gets broken down and little chunks (antigens) go on the outside of the cells. The chunks are held in place by receptors. T cells from the other side of the immune system come by and check out the chunks and decide what to do about them. If the T cells decide the stuff the APC’s eat is bad stuff, they can mobilize their own army.
The Adaptive Immune System
Cells in the adaptive immune system:
- B cells: these make different types of antibodies (IgM, IgA, IgG, IgE)
- T cells
- T Helper cells (CD4+) [TH1, TH2, TH17…]: these release cytokines to stimulate other parts of the immune system.
- Cytotoxic T cells (CD8+): these kill infected or cancer cells
- T regulatory cells: these shut off the immune system after it is done fighting off infection.
The adaptive immune response has memory. B cells make antibodies. These are Y shaped proteins, similar to kitchen tongs. Antigens fit into the groove at the top of the Y, but they have to fit perfectly like a lock and key. Each B cell has randomly generated instruction on how to make their antibodies. There are endless combinations from all the B cells, but each individual B cell makes only one shape of antibody. Each B cells puts some antibodies out on it surface, ready to grab only an antigen that fits perfectly in the groove. Some B cells never find anything that fits into the groove. Once in a while, there is a perfect fit. This mother B cell gets excited and starts to divide (it needs chemicals from the T cells to do this). Some of the daughter B cells turn into plasma cells and start releasing antibodies, IgM type. Some daughter cells hang back and form memory cells, ready to go back to work if they find any more of this particular type of antigen. When the B cells are re-exposed to the antigen, they can switch from making IgM to making IgG, IgE, and IgA antibodies. IgA protects mucus membranes, such as the GI tract, lungs, eyes, ears, and nose. IgG protects the entire body and blood. IgE protects the body from parasites. B cells have to be stimulated every so often or they will die off, this is called anergy. More about anergy later. This section should help you understand vaccine titers too, they are measurements of IgM and IgG antibodies.
T cells also have receptors that are similar to B cell antibodies. They are randomly generated. There are a few major categories of T cells.
- Cytotoxic (CD8+) T cells are supposed to kill humans cells, either cancer or virus infected cells.
- Helper (CD4+) cells release chemicals (cytokines) to help the B cells, other T cells, or macrophages. There are different types of T Helper (TH) cells: TH1 and TH2 are the most well known. Basically, TH1 helps the macrophages and TH2 helps the B cells make antibodies.
- Regulatory (CD4+CD25+FoxP3+) or Treg cells shut off the immune system when the fight is over, and to stop T cells that react to our own cells. Some people with allergic and autoimmune disease don’t have enough Treg cells.
Unlike the B cells, T cell receptors cannot just grab onto antigens, they must get them from an MHC receptor. There are 2 types of MHC, 1 and 2. Every cell has MHC-1, and MHC-1 grabs proteins from inside the cells. The T cells are trained in the thymus to recognize our own MHC and also not to react to our own proteins in the MHC. This is useful because if another human’s cells are in your body, the T cells will kick them out (transplant rejection). And while the T cells will ignore your own protein in the MHC, if the MHC have something in then that doesn’t belong, like virus chunks, the T cells will attack those cells and get rid of them. Cytotoxic T cells read MHC-1 receptors.
MHC-2 is found on APC’s, such as macrophages. Since the job of the APC is to eat stuff, it’s good at finding bad stuff. The Helper cells recognize MHC-2 and will release chemicals (cytokines) when they find something bad in the MHC-2. The cytokines will trigger the B cells and macrophages. Some types of cytokines are interleukin (IL), interferon (IFN), tumor necrosis factor (TNF), and transforming growth factor (TGF).
If something goes wrong and the T cells react to a protein your body makes, like insulin, the insulin cells will get attacked and die. Maybe a virus revved up the T helper cells to release cytokines to trigger the immune system. Insulin is an innocent bystander. Before long, you end up with type 1 diabetes. This is where the Treg cells come in. They are like an internal affairs bureau catching a bad cop.
Well, I didn’t put in all the details I possibly could. If you see I left something major out or made a mistake, please let me know. I am going by memory and fact checking with Wikipedia:)