The response to antigens is a highly coordinated process that uses the many types of cells of the immune system.

Most cells of the immune system are lymphocytes, which are 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. The 2 main groups of antigen-presenting cells are:

• monocytes and macrophages
• dendritic cells

Other types of white blood cells, known as granulocytes, also make up an important part of the immune system, although they are not discussed further here.

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. B cells then move to the lymph nodes, which are bean-sized collections of immune system cells found throughout the body. B cells also collect in some internal organs such as the spleen.

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.

When a B cell comes into contact with an antigen (on a germ or cancer cell), it turns into a plasma cell. Plasma cells release antibodies that recognize and 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 are much like antibodies. These proteins allow T cells to recognize and react to parasites, cancer cells, and cells infected by viruses.

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.

Natural killer (NK) cells

Lymphocytes called natural killer (NK) cells are not as specific 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 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 them. Both monocytes and macrophages can act as APCs to help start an immune response.

Dendritic cells

Like monocytes and macrophages, dendritic cells work by finding unwanted cells in the body, chewing them up, and presenting their antigens on their surfaces. They then travel to an area with many lymphocytes, such as the lymph nodes or spleen. Here, they activate specific 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 in development.

Revised: 03/18/2008