Science in More Detail

Bystander Effect

There are two types of bystander effects when we’re discussing radiation. Both are at a cellular level. Both are ways that cells not touched by radiation, but near it, can be affected by that radiation anyway.

The first type of bystander effect is based on studies suggesting that when a cell within a group of cells is “hit” by a radiation wave or particle and killed, that cell releases chemicals into the area where all the cells are sitting. Those chemicals can either offer a protective or a harmful effect on the cells that are left. This implies that the remaining cells are somehow affected by a cell close by that dies, even though they were not hit by the radiation. The exact mechanism is still not known. If, in some instances, a protective effect occurs, this means the cells would be more resistant to radiation damage. If, in some instance, a harmful effect occurs, this means the cells would be more sensitive to radiation damage. Depending on the type of cell, the type of radiation, and the radiation dose, either of these effects could occur.

The second type of bystander effect is based on studies suggesting that cells close together signal each other, causing other cells to protect themselves or to die. Some studies have shown that in a group of cells that are clustered together, causing the death of one cell by radiation can cause the death of other cells very nearby. They have to be clustered very closely, though, because the same effect is not seen if they are even a cell width apart from each other. And, again, the exact mechanism for this effect is still not known. So, as with the first bystander effect discussed, we might have a positive effect (a signal to other cells to protect themselves) that makes cells more resistant to radiation effects or we might have a negative effect (a signal to other cells to die).


This is a good time to discuss the concept of hormesis (hor-mee-sis), a generally favorable biological response to low exposures to toxins or stressors that will give an unfavorable response at high exposures. There have been some studies of worker populations, plants, animals, and cells that have shown favorable health outcomes at low exposures of radiation as compared to adverse outcomes at high exposures. However, these studies have not been accepted as proof of a hormetic effect from radiation. As one example, there have been some studies in which the authors report that cells exposed to a small amount of radiation (called a conditioning dose) can actually produce what they refer to as an adaptive response that makes cells more resistant to another dose of radiation. Some potential issues with this are (1) that many of the results cannot be reproduced (meaning that other scientists have tried to do the same testing and get the same results, but haven’t been able to; this suggests that the initial results might have been just due to chance), (2) that not every type of cell has this capacity for an adaptive response, and (3) that the adaptive response does not appear to last long (so the second radiation dose would have to occur soon after the conditioning dose). So, is it real or not? Scientists can’t agree because the results of studies are so inconsistent.

Genomic Instability

We’re going to add one more cellular concept—genomic instability. First, the genome of a cell is the DNA-containing portion of the cell. The DNA is what contains our genetic information that is used for the development and functioning of the cell, tissue, or organ that contains the DNA. Thus, as you can probably tell, the DNA is very important to how we function. Genomic instability refers to a situation that can occur if the same genome is somehow changed time and time again. These changes could be the result of radiation interaction, chemical exposure, or another agent. The idea is that the genome is changed once and nothing happens; then in a year or so, it is changed again and nothing happens; then in another year or so, it is changed again and this time it becomes unstable. The number of changes required to make the genome become unstable varies, but the take-away point is that a change can take place and maybe nothing will happen for years to come until another change to that same genome occurs. What does this mean in terms of radiation being harmful? If radiation interacts with a genome and isn’t that “final” change to make it become unstable, then it is likely nothing will happen. If radiation interacts with a genome and it is the “final” change, then it might be the initiating event for a tumor. We never know, though, whether it is the first change or the final change.