Cancer is a disease caused by changes in DNA (mutations). When cells are functioning normally they are required to replicate as part of life, such as: anytime tissue is damaged cells around the area quickly begin replication, the bone marrow frequently has to create new blood cells to fight infections and replace older cells and even normal growth relies on cells to replicate. More on cell replication here.
All healthy cells rely on a variety of different signals to tell them when to replicate, for some cells this could be hormones or cytokines, all of these signals are grouped into the term “growth factors”. These growth factors can be released in a variety of different events, platelets can release cytokines to promote regrowth and healing, various glands can produce hormones in response to age that promote growth in specific cell types. Growth factors often cause a production of cyclins within the cell, there are a variety of different cyclins that activate different cyclin dependent kinases. Cyclin dependent kinases (CDKs) are present in cells for the majority of the cell cycle but are only active once cyclins are introduced in interphase. CDKs catalyze a variety of cellular processes that move the cell through the different phases of interphase and check for DNA damage, space and more. More on the specifics of the checkpoints mentioned here:https://pmc.ncbi.nlm.nih.gov/articles/PMC4990352/. However when DNA is damaged and improperly or not at all repaired, cells can begin to ignore normal functioning.
Mutations often won’t change anything about cell function, as large swathes of DNA are currently thought to have no purpose this DNA is sometimes referred to as “non-coding” or “junk DNA”. Other mutations can change small sections of DNA leading to a few dysfunctional proteins, perhaps the cell will die by not performing the tasks it needs to in order to survive. 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 cause the telltale lumps associated with cancer. These lumps (usually referred to as tumors) are bunches of cells with damaged DNA replicating more than necessary, frequently making clones of themselves. See mitosis for an overview of replication.
The nature of mutation is unpredictable, some cells may be misinterpreting 1 signal as another, some may be overreacting to the usual signals, others may be acting as if they are constantly receiving growth factors, some can 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 and 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 them to mutate further and begin displaying different proteins. These markers means immune cells have no way of discerning between normal cells and mutated cells. This makes cancer all the more dangerous, not only because the immune system won’t attack it but because the immune system causes most symptoms people use to identify illness. Symptoms such as coughing, sneezing, fatigue and fevers, are all symptoms that can cause concern, diseases without these symptoms such as cancer can often be difficult to find or diagnose. More on the immune system HERE
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 tumor and have begun to form another tumor 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 tumor causing secondary tumors. The process of cancer spreading and forming tumors away from the primary tumor is known as metastasis. This is usually the major challenge when treating cancers as tumors 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. Treatments are addressed in more depth later in the website
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 up to medical textbooks. Tumors 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 tumor is.