Updated March 26, 2015.
Immune System Overview

The immune system is made up, in part, of a network of immune cells that form in the bone marrow from a very basic type of cell called a stem cell. Many different types of immune cells can be made from stem cells.

Immune cells circulate in the blood or in a network of channels similar to blood vessels called the lymphatic system. They also congregate in special areas of the lymphatic system called lymph nodes.

Some immune cells have very specific functions. Others have general or non-specific functions. T lymphocytes (T cells) and B lymphocytes (B cells) are examples of specific immune cells.

Each T cell and B cell recognizes and is activated by a single substance. This single substance is called the T cell's or B cell's antigen. When a T cell or a B cell recognizes its antigen and is activated, it makes many identical copies of itself. Each copy recognizes the same antigen as the original T cell or B cell.

T Cells, B Cells, and APCs

There are two main types of T cells. Cytotoxic T cells identify and kill cells that contain the antigen they recognize. Helper T cells release chemical messengers called cytokines that recruit other immune cells to the site of attack. Helper T cells also help cytotoxic T cells do their job.

B cells make antibodies. Each B cell makes only one type of antibody, which is directed against its specific antigen. Just as helper T cells help cytotoxic T cells do their job, helper T cells help stimulate B cells to make antibodies.

Antibodies specific for an antigen on a cancer cell can attach to the antigen and, by several indirect mechanisms, cause the cancer cell?s death.

The immune system also contains antigen-presenting cells (APCs). APCs sample their surrounding environment, eating whatever they come across, and then they display little bits of what they have eaten on their surface. Macrophages and dendritic cells are examples of APCs.

Macrophages patrol the body, eating dead cells, debris, viruses, and bacteria. Dendritic cells are more stationary, monitoring the surrounding environment from one spot, such as the skin. Lymphocytes (T cells or B cells) that "meet" an APC can look at the APC cell surface and see if their specific antigen is present. If their antigen is present, the lymphocytes become activated.

Both T cells and B cells can be activated in immune responses against cancer treatment or cancer preventive vaccines. With preventive vaccines against infectious agents that cause cancer, the activated B cells may produce antibodies that bind to the agents and interfere with their ability to infect cells. Because the agents must infect cells to make them cancerous, this lowers chances that cancer will occur.

_{This article was reproduced from the web site of the National Cancer Institute.}

Immune System Basics Immune System Overview

The immune system is made up, in part, of a network of immune cells that form in the bone marrow from a very basic type of cell called a stem cell. Many different types of immune cells can be made from stem cells.

Immune cells circulate in the blood or in a network of channels similar to blood vessels called the lymphatic system. They also congregate in special areas of the lymphatic system called lymph nodes.

Some immune cells have very specific functions. Others have general or non-specific functions. T lymphocytes (T cells) and B lymphocytes (B cells) are examples of specific immune cells.

Each T cell and B cell recognizes and is activated by a single substance. This single substance is called the T cell's or B cell's antigen. When a T cell or a B cell recognizes its antigen and is activated, it makes many identical copies of itself. Each copy recognizes the same antigen as the original T cell or B cell.

T Cells, B Cells, and APCs

There are two main types of T cells. Cytotoxic T cells identify and kill cells that contain the antigen they recognize. Helper T cells release chemical messengers called cytokines that recruit other immune cells to the site of attack. Helper T cells also help cytotoxic T cells do their job.

B cells make antibodies. Each B cell makes only one type of antibody, which is directed against its specific antigen. Just as helper T cells help cytotoxic T cells do their job, helper T cells help stimulate B cells to make antibodies. Antibodies specific for an antigen on a cancer cell can attach to the antigen and, by several indirect mechanisms, cause the cancer cell?s death.

The immune system also contains antigen-presenting cells (APCs). APCs sample their surrounding environment, eating whatever they come across, and then they display little bits of what they have eaten on their surface. Macrophages and dendritic cells are examples of APCs.

Macrophages patrol the body, eating dead cells, debris, viruses, and bacteria. Dendritic cells are more stationary, monitoring the surrounding environment from one spot, such as the skin. Lymphocytes (T cells or B cells) that "meet" an APC can look at the APC cell surface and see if their specific antigen is present. If their antigen is present, the lymphocytes become activated.

Both T cells and B cells can be activated in immune responses against cancer treatment or cancer preventive vaccines. With preventive vaccines against infectious agents that cause cancer, the activated B cells may produce antibodies that bind to the agents and interfere with their ability to infect cells. Because the agents must infect cells to make them cancerous, this lowers chances that cancer will occur.

_{This article was reproduced from the web site of the National Cancer Institute.}