The Immune System and Cancer Vaccines: Cancer vaccines, whether to prevent or treat cancers, take advantage of the immune system's ready-made network of cells and organs that work together to defend the body against attack by "foreign" invaders. When a foreign organism or entity (an antigen) enters the body, a variety of immune cells and molecules mount an appropriate response. Lymphocytes are one of the main types of immune cells. The most common lymphocytes, B cells and T cells, patrol the blood and lymph system for foreign antigens. Cancer vaccines currently under development are designed to stimulate primarily B or T cells to mount an attack against foreign entities.

The job of the B cell is to ambush antigens circulating in the bloodstream. Protruding from its surface are antibody molecules poised to bind foreign agents such as toxins, bacteria or viruses. Only specific antigens will bind to each B cell.

Once the triggering antigen binds, the B cell digests it, and displays antigen fragments with the help of a marker molecule on the cell surface.

The antigen/marker combination is recognized by a T cell, which in response secretes lymphokines.

These, in turn, stimulate the multiplication and maturation of B cells into plasma cells. The plasma cells pour millions of identical antibody molecules into the bloodstream.

These antibody molecules ambush and inactivate the matching antigens in the blood.

While B cells attack soluble antigens in the body's fluids, the job of the T cells is to attack cells that have either been infected by viruses or altered by cancer.

Cells that display pieces of foreign antigens on their surface with the MHC molecule are called antigen presenting cells (APC). APCs are either B cells, macrophages or dendritic cells. T cells recognize antigens only when they are bound to cell-membrane proteins called major histo-compatibility complex (MHC) molecules. Cells that display pieces of foreign antigens on their surface with the MPH molecule are called antigen presenting cells (APC). APCs are either B cells, macrophages or dendritic cells. T cells recognize antigens only when they are bound to cell-membrane proteins called major histo-compatibility complex (MHC) molecules.

When a T cell is activated by binding to the antigen/MHC complex, it secretes lymphokines. The secreted lymphokines cause several reactions.

Some Lymphokines spur the growth of Memory T cells. Memory T cells are primed to fend off additional attacks by the same antigen. These have a longer lifespan in the body and respond more readily to a second antigen assault than the original T cell. Some T cells become natural killer T cells, known as cytotoxic T cells (or CTL), which attack infected or abnormal cells directly.

Some T cells become natural killer T cells, known as cytotoxic T cells (or CTL), and attack infected or abnormal cells directly. The lymphokines also attract other immune cells, such as macrophages and neutrophils, to the site of the foreign invasion.

Treatment Vaccines: The strategy that researchers use for treatment vaccines involves injecting cancer-specific antigens into patients. The hope is that these antigens will stimulate B and T cells in the immune system to attack cancer cells without harming normal cells.

Prevention Vaccines: Prevention vaccines, on the other hand, are given to healthy individuals to stimulate the immune system to attack cancer-causing viruses and prevent viral infection. Non-infectious viral proteins from cancer-causing viruses are commonly used as antigens to stimulate the immune system for prevention vaccines. This is the same strategy used for vaccines for polio or measles.