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When x-rays or gamma rays interact with human tissue, we must consider the mechanisms of interactions that take place, and what the detriment is to the tissue. To begin that conversation, we must lay down some ground work for the discussion. This includes, among many things, the understanding that human tissue is comprised of many millions of cells, containing organic and inorganic compounds comprised of water, minerals, proteins, nucleic acids, and lipids.

Also, the human body is exposed to and has reparative strategies in place to deal with damage to tissue from radiation exposure. It has been estimated that our bodies respond to approximately 15,000 radiation damage events every second due to natural sources of radiation. (3)

The biological effects of radiation result mainly from damage to the DNA, which is the most critical target within the cell; however, there are also other sites in the cell that, when damaged, may lead to cell death. (3)
Direct and indirect radiation interactions

Direct:
The radiation interacts directly with the critical target in the cell. The atoms of the target itself may be ionized or excited through Coulomb interactions, leading to the chain of physical and chemical events that eventually produce biological damage.

Indirect:
The radiation interacts with other molecules and atoms (mainly water, since about 80% of a cell is composed of water) within the cell to produce free radicals, which can, through diffusion in the cell, damage the critical target within the cell.

Degree of Potential Radiation Injury:
Lethal damage, which is irreversible, irreparable and leads to cell death.

Sublethal damage, which can be repaired in hours unless additional sublethal damage is added that eventually leads to lethal damage.

Potentially lethal damage, which can be manipulated by repair when cells are allowed to remain in a non-dividing state.