The key to a healthy immune system is its remarkable ability to distinguish between the
body's own cells, recognized as "self," and foreign cells, or "nonself." The body's
immune defenses normally coexist peacefully with cells that carry distinctive "self"
marker molecules. But when immune defenders encounter foreign cells or organisms
carrying markers that say "nonself," they quickly launch an attack.
How The Immune System Works:
"The immune system is composed of three "layers" or mechanisms that protect
humans from disease.
- The first line of defense, the skin and mucous membranes, acts as a physical
- The second layer is the innate immune system, a broad-acting, short-term, nonspecific
immune response to pathogens such as bacteria or viruses.
- Microbes that evade the innate system encounter a third layer of protection; a
second, more powerful immune mechanism called the adaptive immune
response. Through adaptive immunity, populations of white blood cells known as
lymphocytes - B cells and T cells - mount a powerful, highly specific immune
system attack on specific pathogens."
This section excerpted from Bio-Medicine
Anything that can trigger an immune response is called an antigen. An antigen
can be a microbe such as a virus, or even a part of a microbe. Tissues or cells from
another person (except an identical twin) also carries nonself markers and act as
antigens. This explains why tissue transplants may be rejected.
The antigens expressed by tumors have several sources:
- Some are derived from oncogenic viruses
- For example, human papillomavirus, which causes cervical cancer
- Others are the organism's own proteins that occur at low levels in normal cells but
reach high levels in tumor cells.
- One example is an enzyme called tyrosinase that, when expressed at high
levels, transforms certain skin cells (e.g. melanocytes) into tumors called
- A third possible source of tumor antigens are proteins normally important for
regulating cell growth and survival that commonly mutate into cancer, inducing
molecules called oncogenes.
How the Immune System Identifies and Eliminates Tumors:
The main response of the immune system to tumors is to destroy the abnormal cells
using killer T cells, sometimes with the assistance of helper T cells. Clearly, some
tumors evade the immune system and go on to become cancers.
Tumor cells often have a reduced
number of specialized molecules
on their surface, thus avoiding
detection by killer T cells.
Some tumor cells also release
products that inhibit the immune
response, for example, by
secreting the cytokine TGF-ß,
which suppresses the activity of
macrophages and lymphocytes.
In addition, immunological
tolerance may develop against
tumor antigens, so the immune
system no longer attacks the
|Image courtesy of the National Cancer Institute
Many cancers (gastric, cervical, colon, liver) are associated with infection and correlate
with the activity of the normal host immune response. Chronic inflammatory conditions
make people more likely to develop certain cancers; for example, patients with Crohn's
disease have a higher incidence of colorectal cancer.
A greater understanding of the ways by which the inflammatory response initiates cancer
may lead to potent new cancer treatments.
This is a term used to describe the action of the immune cells, including T cells, as they
move through the body and look for any abnormalities. When cells become mutated,
they may appear to the immune cells as abnormal. The body then recognizes them as
non-self or foreign.
By eliminating cells that have become abnormal, the immune system helps to protect
against cancer. However, if the cells mutate enough so that they are able to escape the
surveillance mechanisms of the immune system, they may continue to reproduce as
cancer cells. The process is a complex version of 'hide and seek' with major
The secret to its success is an elaborate and dynamic communications network. Millions
and millions of cells, organized into sets and subsets, gather like clouds of bees
swarming around a hive and pass information back and forth in response to an infection.
Immunotherapy and cancer vaccines
Attempts to provide the immune system with the signals that it needs to recognize the
cancer cells as abnormal. If successful, these strategies may allow the body to recognize
and destroy cancer cells, even those that have been able to form a tumor.
The image depicts manufactured antibodies zeroing in on a large cancer cell.
The antibodies will attach to antigens on the cancer cell and dispense a drug to kill it.
|Image provided by CISN archives. All rights reserved.
At every level in its life, a cancer cell and its daughter clones must evade the immune
system. The immune system is a remarkably adaptable system that seeks out and
destroys foreign and harmful agents within an organism.
Cancer cells have developed several ways to evade the surveillance of the immune
system. Many cells have lost proteins (antigens) on their cell surface so that the body
can't recognize that these cells are foreign. Other cancer cells secrete cytokines such as
transforming growth factor beta (TGFß), which inhibit the function of the immune
- Cancer cells send signals to the immune system that, in turn, support or suppress
- In particular, macrophages (a type of white blood cell that surrounds and kills
microorganisms), removes dead cells, and stimulates the action of other immune
system cells, seem to play an important role in cancer.