Immune system | MS Explained | MS Trust - Information, education, research and support.
The immune system is the body's main defence system against infection. It consists of a complex collection of special cells and chemicals that patrol the body, identifying and fighting off material that it identifies as foreign, such as bacteria and viruses.
When a cell is attacked by a virus or other invader, it sends out a chemical as a warning signal. This alerts a type of white blood cell called macrophages which mount the initial immune response. When macrophages encounter foreign matter they encircle and digest it.
When macrophages encounter foreign matter they encircle and digest it
Once some of the invading germs have been destroyed, macrophages use particles of the debris, called antigens, to tell other immune system cells which cells to attack and to encourage a greater response to the invasion. When this happens, macrophages are known as antigen presenting cells 2.
Macrophages use particles of the debris, called antigens, to tell T-cells which cells to attack.
The cells that are prompted to respond are a type of white blood cell called lymphocytes.There are two main groups of lymphocytes
B-lymphocytes or B-cells, which mature in the bone marrow, are programmed to recognise a specific virus or bacterium.
T-lymphocytes or T-cells, which mature in the thymus, respond to the antigen presenting cells.
There are different types of T-cells: helper T-cells help to influence how the immune system responds, whilst killer T-cells (also known as cytotoxic T-cells) attack and destroy cells.
Prompted by the antigen presenting cells, helper T-cells become activated and direct other immune system cells by producing 'messenger' molecules, called interferons, which tell other elements in the immune system how to act.
At the start of an immune response, one of the molecules produced is gamma interferon which stimulates B-cells and killer T-cells to attack 3.
Helper T-cells produce gamma interferon to stimulate B-cells and killer T-cells
Killer T-cells attack and destroy cells which contain the antigen – this includes both the invading cell and any of the body's cells which have been infected. This prevents the germ from reproducing and infecting other cells. B-cells are tuned to specific invaders. When the germ is found in the body, the B-cell clones itself and produces millions of antibodies against that germ. Antibodies are proteins that lock onto the surface of the specific invading germ helping the body to kill it off4
B-cells produce millions of antibodies, which lock onto an invader, helping the body to destroy it.
The immune response causes inflammation in damaged or infected tissue. Inflammation causes local blood vessels to dilate, which increases blood flow to the injured site and allows more white blood cells to attack invaders in the affected area.
Once the infection is under control, helper T-cells release a messenger molecule called beta interferon that helps to calm down the immune response.
Once an infection is over, some of the antibodies developed to fight it remain in the immune system. This creates an 'immune memory', which means that should the same organism invade again, the body is already prepared to combat it. This is why many infectious diseases such as mumps or chicken pox usually only occur once. Vaccination uses this principle to forewarn the immune memory. A small amount of matter from a weakened version of an infection is injected into the body. Whilst this is not strong enough to cause illness, it allows the immune system to recognise the disease and to fight it off should it appear again.
Why does a white blood cell not attack every cell in the body?
The antigen presenting cells are controlled by genes called the Major Histocompatibility Complex (MHC) or the Human Leukocyte Antigen (HLA). These genes, which are different in every person, allow the immune system to indentify which cells belong to the body and which are 'foreign'.
It is these genes that create the problems with rejection in organ transplantation. Unless the genes of the donor and the recipient are closely matched, the immune system will treat the new organ as an invader and start to attack it.
In some conditions, for reasons that are unknown, this protective mechanism fails to work. The immune system turns on the body's own tissue and attacks it as if it were an invader. These conditions are called autoimmune diseases. How this causes the symptoms of MS will be explored in more detail later in this book.