of these antigens as a beacon that attract components of the immune system, much like a great white shark attracted to the scent of blood in the water, which them prepare for attack.
In some cases, the body’s own immune system, through this mechanism of action, is capable of destroying abnormal cellular development and even cancerous cells before they become established in the body. However, after a cancerous cell reproduces or forms a mass, the body’s immune system is often unable to halt its progress.
A number of cancers have been identified through their tumor antigens,11 including malignant melanoma and bone cancer. Individuals with such types of cancer typically produce antibodies against those specific tumor antigens. Nevertheless, the body’s immune system is more capable of destroying cancerous cells early on in some types of cancers such as corneal carcinoma (cancerous tumor development in the uterus from parts of the developing embryo).
Figure 2-14 Cancer cells.
Blood tests can determine the presence of tumor antigens. Antigens are also commonly defined as tumor markers. Measuring tumor markers is a way to screen individuals with no previous symptoms of cancer, and are often capable of aiding in diagnosis and evaluating treatment responses.
■How do Cells Function?
Cells have the ability to move things from one point to another. One major function of cells is the ability to engage in transportation of vital components necessary for cellular health and wellness. Two specific types of transportation are common to all cellular function:
•Active transport
•Passive transport
Figure 2-15 Passive and active transport.
Active transport is defined as something that occurs when the cell must utilize energy for transportation. This involves literal movement of molecules across a cellular membrane.
Because the inside and outside of cells are composed of different substances, cells are often required to work very hard and use energy to maintain adequate balances of molecules and ions inside and out.
One of the most common forms of active transportation occurs across the cellular membrane. Literally thousands of proteins are embedded into the lipid bilayer of the cell. These proteins are likened to workhorses and are positioned in such a way that one portion of such proteins remains on the inside and the other portion of the protein protrudes to the outside of the cellular membrane. Only when the bilayer is crossed are the proteins capable of moving ions and molecules into or outside of the cell.
However, these proteins, often called membrane proteins, are not created equal. For example, one proteins may only be able to move glucose, while another is responsible for the movement of calcium. Hundreds of membrane proteins are found in the body, each charged with a specific function.
When pressure or concentration differs between the inside and outside of the cell (especially in the case of neurons), the proteins are working “against” concentrated gradients. This is typical when an ion wants to move from an area of lower concentrations to an area of higher concentrations. In this situation, membrane proteins must consistently push ions into or out of the cell in order to prepare the neuron membrane to transmit an electrical impulse.
Passive transportation is another cellular function. While active transport relies on energy, passive transport is achieved through several different methods:
•Osmosis
•Filtration
•Diffusion
Osmosis is defined as a process by which molecules pass from semi-permeable membranes with less concentrated solutions into an area of higher concentration. This results in equalization of concentration on both sides (inside and outside) of the membrane.
Figure 2-16 Osmosis and reverse osmosis.
Osmosis is primarily defined as a water-based movement. The balance of ions inside and outside of the cell needs to be the same. If ions are not balanced, water molecules want to enter, which can cause swelling and literally explode or pop the cell.
Diffusion defines a process where molecules move from an area of higher concentration to lower concentration. Think of the unloading of passengers from a train. A high volume of passengers surge from the train car and disperse throughout the train station, decreasing the volume of passengers in the train.
Filtration is another method of passive transport. Most of us are familiar with filters, which function in a physical capacity. Filtration, pressure and other forces such as gravity can propel movement. One example is cardiovascular function, where the heart’s pumping capacity effectively regulates blood pressure as it flows through blood vessels.
As we have seen, cells and cell structures share a number of similarities, and the same applies to the longevity of a cell. We conclude this overview of cytology with an explanation that defines the two main types of ‘death’ experienced by cells.
Cellular necrosis and apoptosis
Necrosis defines cellular death caused by unexpected damage to the cell that can be caused by a number of sources:
•Exposure to toxic chemicals
•Trauma
•Blocked blood flow
•Radiation
In such cases, the cell literally ruptures or pops like an overfilled water balloon, allowing the interior components of the cell to disperse into the cellular environment. Once there, the remnants are consumed by phagocytes.
Figure 2-17 Cellular necrosis
Apoptosis is ‘normal’ cellular death; think of it as an aging cell that eventually wears out. The cell begins to lose its structural shape and further breaks down into smaller bodies.
Some studies have explored the potential of some vaccines to trigger apoptosis in mouse livers.12 Studies attempting to determine whether some cytotoxic anti-cancer agents actually trigger and influence apoptosis have been performed since 2000,13 resulting in improved cancer treatments.
The National Cancer Institute has developed apoptosis inducers,14 which actually cause cancer cells to succumb to apoptosis. Cancer cells have the unique ability to resist apoptosis, but angiogenesis inhibitors and delivery of monoclonal antibodies15 that dispense toxic molecules into cancer cells has shown positive progress in the treatment of different types of cancers today.
Apoptosis is often called a programmed cellular death and is natural. Apoptosis does not cause a cell to ‘burst’ as with necrosis, but rather allows the cell to be consumed and eventually removed.
■Conclusion
