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MS ExplainedThe immune system

How does the immune process work?

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. 1

When macrophages encounter foreign matter they encircle and digest it

1. 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

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 another type of white blood cell called lymphocytes. There are two main groups of lymphocytes, named after the part of the body where they are produced

  • T-lymphocytes or T-cells, which develop in the thymus, respond to the antigen presenting cells
  • B-lymphocytes or B-cells, which develop in bone marrow, are programmed to attack a specific virus or bacterium

There are different types of T-cells - helper T-cells help to influence how the immune system responds whilst killer T-cells attack and destroy cells.

Prompted by the antigen presenting cells, helper T-cells activate and direct other immune system cells by producing 'messenger' molecules called interferons. These molecules tell other elements in the immune system how to act.

At the start of an immune response, a molecule called gamma interferon is produced that stimulates B-cells and killer T-cells to attack. 3

Helper T-cells produce gamma interferon to stimulate B-cells and killer T-cells

3. Helper T-cells produce gamma interferon to stimulate B-cells and killer T-cells

Killer T-cells attack and destroy cells containing the antigen - both the invading cell and the body's own cells that have been infected, preventing the germ from reproducing and then infecting other cells.

B-cells are tuned to specific germs. When the germ is found in the body, the B-cell clones itself and produces millions of antibodies. Antibodies are proteins that lock onto the surface of the specific invading germ helping the body to kill it off. 4

B-cells produces millions of antibodies, which lock onto an invader, helping the body to destroy it

4. B-cells produces millions of antibodies, which lock onto an invader, helping the body to destroy it

The immune response causes inflammation of damaged or infected tissue. Inflammation causes local blood vessels to dilate, increasing blood flow to the injured site and allowing more white blood cells to attack invaders in the affected area.

Once the infection is under control, helper T-cells release different messenger molecules called beta interferon that help 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 the '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 each 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 not known, 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 the next sections.

Next page - What happens in MS?