This section assumes knowledge from the sections: Basic biology, DNA, Mitosis and meiosis, Protein synthesis, and Human cells.
Cancer is a disease caused by changes to DNA, changes in DNA are called mutations.
Mutations often will not change anything about the organisms function as a whole. Large swathes of DNA have very little impact on cell function, sometimes referred to as non coding DNA. Cells within the human body also specialise and perform specific roles, damaging DNA not involved in that cell will likely have very little impact. Other mutations could change small sections of DNA leading to a few dysfunctional proteins. Perhaps mutations could cause the cell to die by not performing the tasks it needs to in order to survive, once again this will have no discernible impact on the organism as a whole due the huge amount of other cells in the human body.
However occasionally a mutation occurs that affects DNA to do with replication, this could cause the cell to replicate far more than usual. Cells replicating more than they are supposed to, can cause the tell-tale lumps associated with cancer. These lumps (usually referred to as tumours) are bunches of cells with damaged DNA causing them to perform mitosis more than necessary.
The nature of mutation is unpredictable. Some cancers may be misinterpreting 1 signal as another, some may be overreacting to the usual signals, or operating as if they are constantly receiving signals, some could stop producing proteins that inhibit or regulate cellular replication (such as previously mentioned cyclins or CDKs). This is why medical professionals describe some cancers as more aggressive, because cancers misinterpreting or ignoring different processes will grow, develop, and spread at different rates.
Cancer is a difficult disease to treat and diagnose due to its complete evasion of the immune system. A detailed overview of the immune system can be found here. Human cells have specific proteins and markers on each cell to ensure immune cells do not attack it. Cancer cells usually have the same markers as other healthy cells because they are descended from healthy cells. Although it is possible for cancer cells to mutate further and begin displaying different proteins. Most cancers will still have these proteins meaning immune cells have no way of discerning normal cells from mutated cells. This makes cancer all the more dangerous, not only because the immune system won’t attack it, but also because the immune system causes most symptoms people use to identify illness. Symptoms such as coughing, sneezing, and fevers, are all symptoms that can cause concern. Diseases without these symptoms such as cancer can often be difficult to find or diagnose.
Note: this is one of the main reasons why cancers are not contagious. If a cancer cell were to enter someone else’s body it would have the incorrect proteins and markers to prevent the new immune system from destroying it because everyone has largely unique protein markers. As the cancer cells are not adapted to battle the new immune system, they would be destroyed without any noticeable symptoms. The only time cancer could be passed on is during organ transplants, as organ transplant patients must have their immune system suppressed so it doesn’t attack the new organ with incorrect proteins. If this new organ has cancer it is possible to spread. However, this is very rare as transplant organs are always checked for viability beforehand.
Cancers can also metastasize, this simply means that cancer cells have travelled from the original tumour and have begun to form another tumour in a secondary site. Usually cancers travel using the vascular or lymphatic system however practically any force or fluid could move cancerous cells away from the main tumour causing secondary tumours. The process of cancer spreading and forming tumours away from the primary tumour is known as metastasis. This is usually the major challenge when treating cancers as tumours that have not metastasized can usually be surgically removed or killed with other localised treatments such as radiotherapy. Cancers that have metastasized often require more complex systemic treatments such as chemotherapy.
Cancer can cause a number of different problems within the human body. Most commonly cancer cells can crowd out healthy cells, taking space and nutrients needed for the functioning cells, this can lead to organ dysfunction. Because most organs have different specialised functions explaining the symptoms of each would be lengthy and will thus be left to medical textbooks. Tumours can also block a variety of cavities that are supposed to remain open such as veins and arteries, leading to organ failures and those subsequent problems. Additionally cancer cells can still produce proteins they originally produced while healthy. Too many of these proteins can cause problems, including blood clots; various proteins that bind with platelets to form clots can be released in excess in cancer patients. All of this to say complications of cancer are vastly different depending on which type and where the tumour is.