Revival: Biological Effects of Low Level Exposures to Chemical and Radiation (1992)

98 BIOLOGICAL EFFECTS OF LOW LEVEL EXPOSURES

Table 5.2. Necrosis vs Apoptosis

Necrosis Apoptosis

Injurious insult +

(hypoxia, toxins, etc.) (programmed)

Geography Local/focal Semirandom

Inflammatory response + —

Morphology

— Nuclear chromatin Flocculates (late) Condenses, fragments

(early)

— Cytoplasm Swells, blebs develop Condenses, organelles

preserved

Executioner Ca+ +-dependent

phospholipases

Ca+ +-dependent

endonuclease

well as B, cells. The two principal categories of regulatory T cells are appro­

priately termed “helper” and “suppressor” cells.

T cells, and possibly B cells, are also unusual in that the vast majority are

destroyed prior to their complete differentiation and release to the periph­

eral lymphoid tissues of the host.11 With T cells, this phenomenon (known

as apoptosis) occurs within the thymus and results in the death in situ of

approximately 90% of developing thymocytes. A similar phenomenon has

been suspected, but as yet not documented, for B cells. In part, failure to

demonstrate this phenomenon with B cells may relate to the observation

that in most rodents, and probably also in humans, the differentiation of B

cells occurs in the same anatomic site as does the generation of pre-B cells

(i.e., the bone marrow), which markedly confounds the interpretation of

experiments designed to address this issue.

RADIATION

The cell types involved in the immune response exhibit a broad spectrum

of radiosensitivities.9 For example, some subpopulations of lymphocytes

are exceedingly radiosensitive, while plasma cells and macrophages are very

resistant. The basis of these differences in radiosensitivity are not well

understood, but presumably relate to the degree of differentiation and

maturation of the involved cell type, as well as to whether or not cell

division is a requisite for participation. In addition, some types of lympho­

cytes die acutely after irradiation (so-called interphase cell death) while

others succumb in more traditional fashion. Interphase cell death is of

particular importance with low-dose exposures and may, in fact, account

for the exquisite sensitivity of the small lymphocyte to low doses of radia­

tion. Interphase cell death is distinctive morphologically and is similar to, if

not identical with, apoptosis.13-15 The features that distinguish apoptosis/

interphase death from necrosis are listed in Table 5.2.