Cancer and the Immune System: Enigma Called Cancer

Cancer and the Immune System: Enigma Called Cancer

AN ENIGMA CALLED CANCER

The word “cancer” is an umbrella term that refers to about 200 diseases that share two common characteristics: first, an uncontrolled growth of cells and second, the ability to invade and damage normal tissues either locally or at distant sites in the body. Since a cell is our body’s basic unit of life, this disease could not have chosen a more effective route to wreak havoc on the entire body. Some human cancers arise in the epithelium (the layers of cells covering the surface of the body and the lining of internal organs and various glands); these cancers are called carcinomas. Sarcomas are cancers of the supporting tissues of the body, such as bone, muscle and blood vessels. Cancers of the blood and the lymph glands are called leukemias and lymphomas, respectively. Gliomas are cancers of the nerve tissue. Melanomas arise from darkly pigmented cells, usually in the skin. There is an imperative to understand and control cancer because to date, apart from heart disease, more people die from cancer than any other disease.

1.1 The genesis of cancer

The number of cells in an average human being is about a hundred trillion (1014) or beyond. Some of them, for example, brain and nerve cells, are not actively dividing while others like the cells of the skin, gut, bone marrow, and sex organs continually undergo rapid cell divisions to replace aging and dead cells. It has been estimated that about one million cells commit suicide every second in the adult human being. on an average day, the human body produces and concomitantly eradicates about 60 x 109 cells; on an annual basis, this enormous amount of cells is equivalent to an entire body weight. In order to replace a dead cell, an existing cell must divide and for each successful cell division, the entire genetic material of the mother cell in the form of DNA must be faithfully copied by enzymes and handed over to the new daughter cells. If we compare this to the task of photocopying the entire content of the encyclopedia Britannica again and again for over a trillion times a day, one can appreciate the number of errors (due to exhaustion) that would begin to appear in each copied volume after only a few hours. Fortunately, our cells are equipped with enzymes that not only copy but also proofread, edit, and correct errors in the newly manufactured DNA that is destined for the daughter cells. As with most things in life, this system is not perfect and errors do get passed these proofreading, editing, and correcting enzymes. Cancer occurs when the DNA sequence within a gene is altered in such a way that the gene can no longer instruct the cell in which it resides to produce the normal version of the protein it encodes. Scientists call such an occurrence a mutation within the gene. Such alterations can occur more frequently when a gene is exposed to ionizing radiation or certain drugs or chemicals or when some, as yet, unexplained internal switch is flipped on or off. These factors can cause the DNA sequence within the gene to break and recombine incorrectly or to mutate. once one of these changes has taken place, certain genes (proto-oncogenes) may be transformed into oncogenes (cancer-causing genes), while other genes (cancer-suppressing ones called anti-oncogenes or tumor-suppressors) may be rendered useless by inactivation. A cell containing mutated genes that result in the loss of its growth control is referred to as a transformed cell. Those who have peered into such a cell report that it contains a veritable gallery of cellular horrors like inactivated genes, extra or missing chromosomes, and a host of other genetic abnormalities that cause cancer. Say, for example the alteration of the DNA is like the ignition of an unoccupied car being turned on. The transformation of a normal gene into an oncogene is like moving the gear from P (Park) to D (Drive) while the handbrake is still on. Inactivation of anti-oncogenes is akin to the release of the handbrake and the car is now free to wander onto a highway and cause mayhem. That is the ultimate fate of a transformed cell. It is important to note that it normally takes multiple mutations before cancer occurs. Mutations of our DNA are occurring constantly due to environmental insult. However, a single uncorrected mutation event will not guarantee cancer, rather multiple mutations are required. This is why, for instance, cancer occurs only after years of exposure to a carcinogen (smoking, sun, asbestos, etc).

If a gene has become an oncogene, the cell in which it is located may begin to produce unusually large amounts of one of its normal proteins or to manufacture an altered form of that protein. If an anti-oncogene has been rendered inactive, the cell containing it can no longer produce a normal protein whose function is to suppress cancer. on some rare occasions, a normal cell becomes cancerous when a particular virus enters the cell and introduces an oncogene into the genome of the host cell. once any of these deviations in normal protein production and/or function has occurred, the size, shape, surface characteristics, or morphology and behavior of the cell becomes altered. Thus, it becomes a cancer cell that is distinguishable from a normal cell