What the immune system does

The immune system is your body's defense force. It helps keep invading germs out, or helps kill them if they do get into your body.

Your immune system is a collection of organs, special cells, and substances that help protect you from some infections and diseases. Immune system cells and the substances they make circulate through your body to protect it from germs that cause infections. They also help protect you from cancer in some ways.

It may help to think of your body as a castle. Think of viruses, bacteria, and parasites as hostile, foreign armies that are not normally found in your body. They try to invade your body to use its resources to serve their own purposes, and they can hurt you in the process. In fact, doctors often use the word foreign to describe invading germs or other substances not normally present in the body.

The immune response

Any substance that raises an alarm in the body, causing the immune system to react to and attack it is called an antigen. This immune response can lead to destruction of both the antigens and anything they are attached to, such as germs or cancer cells.

Germs such as viruses, bacteria, and parasites have substances on their outer surfaces, such as certain proteins, that are not normally found in the human body. The immune system sees these foreign substances as antigens. Cancer cells are also different from normal cells in the body. They often have unusual substances on their outer surfaces that can act as antigens.

But the immune system is much better at recognizing and attacking germs than cancer cells. Germs are very different from normal human cells and are seen as truly foreign, but cancer cells and normal cells can be very much alike, with fewer clear cut differences. Because of this the immune system may not always recognize cancer cells as foreign. Cancer cells are less like soldiers of an invading army and more like traitors within the ranks of the human cell population. This may be why cancers are often able to grow in spite of a healthy, working immune system.

Key players in your immune system

Your immune system responds to antigens in a highly coordinated process that uses many types of cells.

Most cells of the immune system are lymphocytes, a type of white blood cell. Several types of lymphocytes work together to attack cancer cells:

• B cells (B lymphocytes)
• T cells (T lymphocytes): killer T cells, helper T cells, regulatory (suppressor) T cells
• Natural killer (NK) cells

Antigen-presenting cells (APCs) are not lymphocytes but work closely with them to fight cancer. They take part of the foreign cell and carry it to where other immune cells can "see" it. This helps stimulate the immune reaction. The 2 main groups of antigen-presenting cells are:

• Monocytes and macrophages
• Dendritic cells

Other types of white blood cells, known as neutrophils or granulocytes will not be discussed in this document, but they also make up an important part of the immune system. Their role is to fight and kill bacteria.

Lymphocytes

B cells and plasma cells

B cells (B lymphocytes) are made in the bone marrow, which is the spongy inner part of some bones. After they are made, most B cells move to the lymph nodes, which are bean-sized collections of immune system cells found throughout the body. B cells also collect in the lymph tissue contained in some internal organs such as the spleen, stomach, and intestines.

B cells can't directly destroy germs or cancer cells by themselves. But they play an important role in immune defenses by making antibodies, which are large, sticky proteins. Each antibody is made to attach to a certain antigen.

When a B cell comes into contact with an antigen (on a germ or cancer cell), it starts making antibodies and turns into a plasma cell. Plasma cells release antibodies that bind (attach) only to that antigen. The antibodies then help kill any cells that have the antigen. The antibodies may destroy them directly or they may serve as a marker for other immune system cells, such as T cells, to destroy them.

T cells

Some lymphocytes that are formed in the bone marrow enter the bloodstream before they are fully mature. They go to the thymus (a small gland in front of the heart and behind the breastbone), where they mature and gain new disease-fighting properties.

Once they leave the thymus gland, they are known as T lymphocytes or T cells (named for the T in thymus). T cells gather in the lymph nodes and spleen, where they work together with other immune system cells. T cells have special proteins on their surfaces that allow them to recognize and react to parasites, cancer cells, and cells infected by viruses, much like antibodies do.

There are 3 main kinds of T cells. They each have different jobs.

• Killer T cells (cytotoxic T lymphocytes) destroy unwanted cells in the body. When these cells come in contact with the specific foreign cells they recognize, they give off substances that kill the cells.
• Helper T cells do not directly kill cancer cells or germs, but they release substances that help B cells and killer T cells work better.
• Regulatory (suppressor) T cells act as brakes to help keep the immune system in check. They help ensure that the immune system does not overreact and attack other healthy parts of the body. These are sometimes called Treg cells.

Natural killer (NK) cells

Lymphocytes called natural killer (NK) cells are not as picky as killer T cells in what they attack. When fighting cancer, they are drawn to areas with cancer cells by substances given off by other cells. They attach to cancer cells and release substances that split the cells open, killing them. They then look for other cancer cells to attack.

Antigen-presenting cells

The main function of antigen-presenting cells (APCs) is to help lymphocytes recognize antigens on foreign cells (including cancer cells). Antigen-presenting cells include monocytes, macrophages, and dendritic cells.

Monocytes and macrophages

Monocytes are made by the bone marrow and released into the bloodstream. Some monocytes enter tissues and organs. Here they become macrophages, capable of surrounding and "eating" unwanted cells. They then present antigens from the devoured cells on their outer surface, so that lymphocytes can recognize the foreign antigens if they are found in the body later on. Both monocytes and macrophages can act as APCs to help start an immune response.

Dendritic cells

Like monocytes and macrophages, dendritic cells find unwanted cells in the body, chew them up, and present their antigens on their surfaces. They then travel to an area with many lymphocytes, such as the lymph nodes or spleen. Here, they activate certain lymphocytes to go out and attack any similar cells in the body. Dendritic cells are not common, but they are the most powerful type of antigen-presenting cell. Because of this, they are the focus of many cancer vaccines currently being developed.