1.     Introduction
In cancer human cells began to divide enormously and spread to the nearby tissues.  It start from the any part and anywhere of the body. Human body are made up of trillions of cells. Generally cells grow and divide to form new cells as per the requirement of the body. With the cells became old and damaged it dies and it is replaced by the new cells. But someone when get cancer this normal process does not happen and abnormal process occur. In this case the old and damaged cells survive and the new cell form when not required. So the division of extra cell occurs without interruption and form masses called tumor.  Most cancers are solid tumors and these are masses of tissue.   In case of Blood cancers (leukemia) solid tumors are generally do not form. . Cancerous tumors are malignant that is they can  which means they can extent to nearby tissues or can invade. Some cancer tumor can spared to distance part of the body via cardiovascular system or lymphatic system and form new tumors far from the original tumor. In some cases the benign tumors could be quite large.  Benign brain tumors are life threatening.
2.     Dissimilarities between cancer and normal cells
Cells of cancer grow widely and are very destructive. The difference between  a cancer cell and a normal cell is that  the cancer cell is less   specified than normal cell. That is cancer cell used to divide enormously. It does not obey the command or signal which normal articulate cell to stop dividing meaning programmed cell death or apoptosis. Cancer cells are capable to stimulus normal cell  and grains in blood vessels and feed the cells of a tumor. This is known as microenvironment.  For instance cancer cells induced normal cell to adjacent blood vessel providing required oxygen and nutrient creating good environment to grow tumor. Cancerous cells are capable evade the resistant system, organs network, tissue and specific cells that guard the body from infections and diseases. Though, ordinarily the immune system of the body eliminates spoiled or anomalous cells. Sometimes cancer cells hide the immune system. The tumors can make the immune system alive and grow.
3.     How cancer appears
Cancer disease initiated through the change in genes.  The genetic alteration which causes cancer may be congenital from paternities. It could also occur as a result of faults id dividing cells or from environmental exposure such as smoke, radiation or ultraviolet rays.  Mostly cancer cells are genetic change for example mutations in DNA than normal cell however some changes may not be linked to cancer. Genetic change tends to effect three types of genes likely 1.  Proto-oncogenes 2. Tumor suppressor genes and 3. DNA repair genes. It is call cause of cancer. The proto-oncogenes are committed to ordinary cell development and division. In any case, when these qualities are modified in certain ways or are more dynamic than typical, they can get to be tumor bringing on qualities (or oncogenes), by permitting the phones to develop and survive when they ought not. Tumor silencer qualities are additionally required in controlling cell development and division. Cells with a few adjustments in tumor silencer qualities can be partitioned into an amorphous control. Cells with transformations in these qualities tend to form extra changes in different talents.
4.     When cancer spreads
A cancer which spread from the root place where from it spread is called the metastatic cancer and the process by which it spared is called metastasis. Metastatic cancer cells usually have the same appearance as that of original cancer cells. Additionally, cells and metastatic cancer cells from the original cancer generally have some molecular features, as the presence of specific changes in the chromosome .Treatment can help lengthen the lives of people with metastatic cancer

5.     Types of cancer
It is said that there are several types of cancer it could be more than hundreds. Its name mostly called where from its organs, tissue  originate such as, lung cancer starts from the lung cells, brain cancer starts from  the brain cells, bone cancer etc.. Cancers can also be designated by cell type that forms, such as epithelial or squamous cell. Here are some categories of cancers
5.1.                     Carcinoma
This is one of the most common kinds of cancer. It start  in epithelial cells, these  cells cover internal and external surfaces of the body. There are several types of epithelial cells, which commonly have a column in microscopic view. Carcinomas which began in epithelial cells have particular names.
5.1.1.       Adenocarcinoma is is formed in the epithelial cells which produce mucus or fluid. Tissues with this type are also called glandular tissues. Mostly  breast, colon and prostate cancers are adenocarcinomas.
5.1.2.      Basal cell carcinoma begins in the lower or basal stratum of the epidermis, this  is the outer stratum of the skin.
5.1.3.      Squamous cell carcinoma forms in squamous cells, these are epithelial cells which lays below the external surface of the skin.  organs like stomach, intestines, lungs, bladder and kidneys Squamous cell carcinomas are also  called epidermis carcinomas.
5.1.4.      The transitional cell carcinoma is a type of cancer that create a type of epithelial tissue called transitional epithelium or urothelium. 
6.      Sarcoma
Sarcomas occur in bone and soft tissue, muscle and , adipose tissue, blood vessels, lymph vessels and fibrous tissue like  tendons and ligaments. most common bone cancer is Osteosarcoma. Common types of soft tissue sarcomas are called as
The Kaposi's sarcoma,
The malignant fibrous histiocytoma
The lip sarcoma
And protuberant dermatofibrosarcoma.
7.     Leukemia
This begins in the tissues which makes blood in the bone marrow. It is called  leukemias. It does not create solid tumors. Instead, here  large quantity  of unusual white blood cells such as leukemia cells and leukemic blasts gather in the blood and bone marrow and displaces normal blood cells. Due to low concentration of normal blood cells causes difficulty for the body to carry oxygen and consequence is that it became unable to control the bleeding or unable to fight with  infections.  Leukemia are grouped according to the type of worsening
Or chronic
And the type of blood cell where the cancer begin
Or myelogenous

8.     Lymphoma
Lymphoma starts in lymphocytes that are T cells or B cells. It is white blood cells.  It  fight with disease and is a  part of the resistant system. In lymphoma unusual lymphocytes gather in the lymph nodes and lymph vessels and other body organs.
9.      Multiple myeloma
Multiple myeloma occur in plasma cells. Unusual myeloma cells gather in bone marrow forming tumors in bones all over the body. this is called plasma cell myeloma too.

10.                        Melanoma
Melanoma is cancer which begins skin however it may also be made in other pigmented tissues such as eyes.
11.                        Brain tumors and spinal cord
Different kinds of tumors of the brain and spinal cord exist .  tumors are named for the kind of cell where they were made and where the tumor originate in the central nervous system like   astrocytes tumor. It is star shaped. .Brain tumors can be noncancerous or cancerous.
12.                        Other tumor types
12.1.                    Germ cell tumors
Is a kind tumor that originates in the cells that make sperm or eggs. This type of tumors can happen anywhere of the body and can be of benign or malignant type.
12.2.                    Neuroendocrine tumors
It is a  formed of cells which  secrete hormones into the blood in response to a signal from the nervous system. These tumors used to produce hormones in larger amounts.  It exhibit  many different signs. Neuroendocrine tumors are either  benign or malignant.
12.3.                     Carcinoid tumors
It is a type of neuroendocrine tumors. these are slow growing tumors and  found usually  in the gastrointestinal tract most commonly in the rectum and small intestine. Carcinoma tumors spread to the liver or even other sites of the body. It can conceal substances like serotonin or prostaglandins initiating carcinoma.

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