How They Start and How They Differ From Normal Cells

By Lynne Eldridge, MD 

Updated on October 21, 2023

 Medically reviewed by Benjamin Leach, MD

Cancer cells differ from normal cells in the body in many ways. Normal cells become cancerous when a series of mutations (changes) leads the cells to continue to grow and divide out of control.

Unlike normal cells that remain in the region where they began, cancer cells have the ability to both invade nearby tissues and spread to distant regions of the body.

This article discusses cancer cells. It explains how cancer cells develop and how they differ from normal cells. It also explains why the body does not recognize and destroy cancer cells as it does other "foreign" cells.

vitanovski / Istockphoto.com

What Is Cancer?

Cancer is a disease that occurs when cells become abnormal and grow out of control. Normal cells grow—and then die—when they are given signals to do so. Cancer cells ignore these signals and continue to multiply. Cancer cells also may form a tumor at the original site and then spread and form new tumors in other places.¹

Types of Cancer Cells

There are as many types of cancer cells as there are types of cancer. Of the hundred-plus types of cancer, most are named for the type of cancer cells in which the disease began.²

Carcinomas

Carcinomas are cancers that arise in epithelial cells that line body cavities. The most common type of cancer cells, carcinomas are named after the type of epithelial cells that mutated. These include:³ 

• Adenocarcinoma: Cancer that affects epithelial cells in glandular tissue, such as the breast, colon, and prostate

• Basal cell carcinoma: Cancer that affects epithelial cells in the basal (lower) layer of the epidermis (skin)

• Squamous cell carcinoma: Cancer that affects squamous epithelial cells, which are found just beneath the outer surface of the skin and in the stomach, intestines, lungs, bladder, and kidneys

• Transitional cell carcinoma: Cancer that affects transitional epithelium (urothelium), which are various-sized, multi-layered epithelial cells in the bladder, ureters, and kidneys

Sarcomas

Sarcomas are cancers that arise in bone, muscle, blood, fat, and other soft tissue cells known as mesenchymal cells. Bone and soft tissue sarcomas include:³

• Dermatofibrosarcoma protuberans: A type of skin cancer

• Kaposi sarcoma: Cancer that forms in the skin, lymph nodes, internal organs, and mucous membranes

• Leiomyosarcoma: Cancer that affects smooth muscle tissue

• Liposarcoma: Cancer that forms in fatty tissues

• Malignant fibrous histiocytoma: Cancer that can affect bone or soft tissue

• Osteosarcoma: A type of bone cancer

Leukemia

Leukemia cells and leukemic blast cells are abnormal white blood cells that form in bone marrow stem cells, where blood cells are made. Unlike some other cancers, leukemia cells do not bind together to form a tumor.

Instead, these abnormal white blood cells build up in the blood and bone marrow, crowding out healthy blood cells. There are four types of leukemia:⁴

• Acute lymphoblastic leukemia (ALL)

• Acute myelogenous leukemia (AML)

• Chronic lymphocytic leukemia (CLL)

• Chronic myelogenous leukemia (CML)

Lymphomas

Lymphomas are blood-related cancers that arise from lymphoid tissues in the lymphatic system, a part of the immune system that runs throughout the body.

Lymphomas can develop in different types of white blood cells known as B-cells, T-cells, and NK cells. They can begin anywhere in the body and feed on nutrients in the lymph fluid.⁵

There are two main forms of lymphoma:³

• Hodgkin lymphoma

• Non-Hodgkin lymphoma (NHL)

Multiple Myeloma

Multiple myeloma is a type of blood cancer that develops in plasma cells—white blood cells that produce antibodies. They occur when a plasma cell becomes abnormal, then divides to replicate, forming myeloma cells.⁶

Myelomas collect in the bone marrow and soft tissue to form a tumor. When it affects several bones, it is known as multiple myeloma. It can also affect other tissues and organs, such as the kidneys.

 Causes and Risk Factors of Leukemia

How Do Cancer Cells Form?

Cancer cells start to form when genes, made up of DNA, experience certain changes (mutations) that cause the cells to behave abnormally. These changes may be inherited or caused by external factors, like smoking or exposure to ultraviolet rays. Mutations can also be completely random.³

National Cancer Institute. What is cancer?

Cancer cells appear through a series of genetic and epigenetic (or environment-induced) changes. Some of these changes may be inherited or, more often, caused by carcinogens (cancer-causing substances) in the environment.⁷

In general, solid tumors contain multiple mutations (also known as variants, which represent a change from the original). Interestingly, the metastatic process that is the main culprit for the high mortality of advanced cancers is thought to be caused mostly by epigenetic changes, as no specific genetic alterations have been found in metastases.⁸

Click Play to Learn About the Start and Characteristics of Cancer Cells

This video has been medically reviewed by Doru Paul, MD

A genetic predisposition to cancer does not mean you will get cancer. However, if a few mutations are already in place, it will likely take fewer acquired mutations for a cell to become cancerous.

The process of normal cells becoming cancer often goes through stages in which the cell becomes progressively more abnormal in appearance. These stages may include hyperplasia (enlarged) and dysplasia (growing abnormally) before cancer.

Sometimes, this process is described as differentiation. Early on, a cell may look much like normal cells of that organ or tissue. As the progression continues, the cell becomes increasingly undifferentiated. This is why sometimes the original source of cancer cannot be determined.

Recap

Cancer cells form when genetic mutations cause cells to behave abnormally. The mutations can be inherited or be caused by exposure to carcinogens like tobacco smoke or ultraviolet rays. In some cases, mutations can occur randomly.³

If you have a family history of cancer, you are in no way "destined" to get it, too. Cancer is a disease that is caused by a combination of factors (multifactorial). Lifestyle and environmental factors will play a role, too.

How Cancer Cells Divide and Grow

A cancer cell can have thousands of mutations, but only a certain number of these genetic changes in cancer cells cause cancer to divide and grow.⁹ Mutations that result in the growth of cancer cells are referred to as "driver mutations," whereas other mutations are considered "passenger mutations."

Normal genes that help cells grow, called proto-oncogenes, can become "oncogenes" (or those with the potential to cause cancer) when mutated and drive the growth of cancer. By contrast, tumor suppressor genes are genes within the cell that tell cells to slow down and stop growing and repair damaged DNA. And they tell cells when to die.

Recap

Most cancer cells have mutations in both oncogenes and tumor suppressor genes, which lead to their behavior.

Cancer Cells vs. Normal Cells

There are many important differences between cancer cells and normal cells:

Cancer Cell

May keep growing

May invade nearby tissues

May spread to other regions of the body

Can be immortal

Normal Cell

Grows when needed

Stays within tissue boundaries

Sticks to nearby cells

Has a defined lifespan

Growth

Normal cells grow during development stages, such as during childhood, or to repair injured tissue. Cancer cells continue to grow (reproduce) even when further cells are not needed.³

Cancer cells also fail to listen to signals that tell them to stop growing or commit "cell suicide" (apoptosis) when the cells become old or damaged. They also ignore signals that tell them to stop dividing and continue to multiply instead.

Ability to Invade Nearby Tissues

Normal cells respond to signals from other cells that tell them they have reached a boundary. Cancer cells do not respond to these signals and extend into nearby tissues, often with finger-like projections.¹⁰

This is one reason why it can be difficult to surgically remove a cancerous tumor.

Ability to Evade

Cancer cells are able to evade detection from the immune system, which normally hones in on and eliminates abnormal or damaged cells.³ 

In addition to their ability to hide, cancer cells can also trick the immune system into protecting tumors instead of attacking them. 

Ability to Spread

Normal cells make substances called adhesion molecules that cause them to stick to nearby cells. Some cancer cells, lacking the stickiness caused by these adhesion molecules, can break free and float to other regions of the body.

These cells may travel to nearby tissue or through the bloodstream and lymphatic system to areas of the body far from the original cancer cell. For example, a lung cancer cell may travel (or metastasize) to the lymph nodes, brain, liver, or bones.¹¹

Immortality

Most normal cells, like humans, have a limited lifespan. When they reach a certain age, they die. Cancer cells, in contrast, have developed a way to defy death.¹²

At the end of our chromosomes is a structure known as a telomere. Every time a cell divides, its telomeres become shorter. When the telomeres become short enough, the cells die. Cancer cells have figured out a way to restore their telomeres so that they don’t continue to shorten as the cell divides. In this way, they become immortal.

Cancer Cells Hide From the Immune System

Our immune system does not recognize and remove cancer cells as they would a bacteria or virus. This is because cancer cells evolve to avoid detection.¹³

Immune cells called natural killer cells have the job of finding abnormal cells and marking them for removal by other cells in our immune system. Cancer cells remain alive either by evading detection (they disguise themselves in different ways) or by inactivating the immune cells that come to the scene.

The ability of the immune system to recognize and eliminate cancer cells is thought to be responsible for the uncommon but well-documented phenomena of the disappearance of some cancers without treatment (like the spontaneous remission of cancer.) This process also lies at the crux of the new field of cancer treatment known as immunotherapy.

The word "cancer" comes from the Greek word carcinos, which means crab. Hippocrates chose this term because he saw the similarity between a crab and the claw-like extensions of cancer (which invade nearby tissue).

Cancer Cells Change

Once cancer has formed, the cells don't remain the same. Rather, continued mutations may occur. This is why resistance develops to chemotherapy and targeted therapy drugs. The cancer cell develops a mutation that allows it to bypass the damaging effects of these treatments.¹⁴

Cancer cells' ability to change is very important in treatment. For example, estrogen-receptor (ER) positive breast cancer may change into ER-negative cancer if it recurs or spreads. This would require a different treatment.

It also helps explain why cancer cells in different parts of a tumor may be different. This is referred to as "heterogenicity" and is important in diagnosis and treatment.

 What It Means if You Have Precancerous Cells

Precancerous Cells

Precancerous cells may look abnormal and similar to cancer cells but are distinguished from cancer cells by their behavior. Unlike cancer cells, precancerous cells do not have the ability to spread (metastasize) to other regions of the body.

An often-confusing condition is carcinoma-in-situ (CIS). It consists of cells with abnormal changes found in cancer cells. But since they have not spread beyond their original location (or technically, have not gone beyond something called the basement membrane), they are not technically cancer. Since CIS can turn into cancer, it is usually treated as early cancer.¹⁵

Summary

Cancer cells appear through a series of genetic and environment-induced changes. The process of normal cells becoming cancer often goes through stages in which the cell becomes progressively more abnormal in appearance. But this is just the beginning.

Cancer cells differ from normal cells in their ability to spread, invade nearby tissue, and "live" on in perpetuity. They can accomplish this by evading detection (disguising themselves in different ways) or by inactivating the immune cells that come to the rescue.

Once cancer has formed, the cells don't remain the same. Rather, continued mutations may occur. This is why resistance develops to chemotherapy and targeted therapy drugs.