Every cancer results from abnormal changes in the genes of a normal cell. Genetic mistakes are common, but the cell usually recognizes and repairs them. Those that escape recognition can replicate and give rise to other, increasingly bizarre genetic mutations. Each cancer is the cumulative product of its mutations and behaves accordingly. While each tumor is individual, all cancers share some characteristic effects on the body.
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Normal cells are inhibited in their growth rate by contact with each other. This has the beneficial effect of an orderly pattern of reproduction and cell replacement. All tumors -- cancerous and noncancerous -- lack that inhibition and grow exuberantly, piling up on one another and producing a swollen mass, or tumor. Tumors can press against normal structures, such as blood vessels and nerves, and produce pain or cause malfunctions. Cancerous tumors have the added capacity to invade and destroy neighboring structures, causing bleeding, blockages, and loss of production of normal hormonal and biochemical products.
Spread to Other Sites
A cancerous tumor is a chaotic place where genetic mutations occur in multiple steps, producing strains of cells that vary in their capabilities. Some mutations are lethal for the cell, but others confer characteristics that enable further misbehavior. The ability to leave the original tumor site and travel to and implant elsewhere in the body -- metastasis -- is acquired by this mutation mechanism. Metastatic tumors invade the new ground, causing damage and producing further mutational variants. A common example is the spread of cancer from breast, lung and prostate to bones, where the cancers cause painful and disabling fractures.
Weight loss occurs in up to 80 percent of people with cancer, according to Dr. Benjamin Tan and colleagues in an April 2011 article in "Journal of Genetics." The lost mass is in both muscle and fat, and results from tumor suppression of appetite and alterations in biochemical pathways for the production of proteins and other compounds. These changes are the result of the mutant gene products from within the cancer interacting with normal organs and tissues.
Decreased Resistance to Infection
The immune system regularly recognizes and destroys mutant cells. Some mutations give the cancer cell the ability to cripple the immune response and escape its control. The impaired immune system is also limited in its capacity to identify foreign organisms, such as viruses and bacteria. Infections may occur throughout the body, particularly in the skin and lungs. Pneumonia is the final complication in many people with cancer.