96 BIOLOGICAL EFFECTS OF LOW LEVEL EXPOSURES
distinct cell types but also a potpourri of factors that they produce. The
involved cells are often grouped together in highly specific spatial relation
ships in a series of so-called lymphoreticular tissues, which include the
thymus, spleen, lymph nodes, and the gastrointestinal tract-associated
lymphoid tissues. In addition, one of the cell types —the lymphocyte —
constantly recirculates throughout the host in highly specific fashion, as
determined by a complex of cell surface molecules and their tissue
equivalents.
The major cellular components of the immune system are lymphocytes,
macrophages, and a series of macrophage-related accessory cells. Lympho
cytes are of critical importance in almost every facet of the immune
response. Among their functions are the recognition of antigen as foreign
(“non-self”), the induction and modulation of the resultant immune
response, and, most particularly, the high degree of specificity of these
responses. Lymphocytes are also responsible for the “memory” that accom
panies most immune responses and the “tolerance” that prevents host cells
from initiating a response directed against host antigens.
The role of the macrophage is less specific but no less critical than that of
the lymphocyte. Whereas individual lymphocytes are precommitted to
respond to a limited number of structurally related antigens, macrophages
appear to be far less discriminating. Thus, at present, they are not thought
to have the same high degree of antigen specificity that characterizes lym
phocytes. Rather, macrophages are responsible for the phagocytosis and
degradation of complex antigens to the more simplified forms that are then
susceptible to recognition by lymphocytes. Macrophages also produce a
number of immunologically active mediators that are capable of regulating
various types of lymphocytes.
Accessory cells resemble macrophages in many ways but appear to be
functionally more sophisticated. Perhaps as a consequence, accessory cells
are often found in highly specific locations in complex lymphoreticular
tissues such as the thymus.
Despite morphologic homogeneity, at least at the light microscope level,
small lymphocytes may be subdivided functionally into two major groups:
- Thymus-derived or T cells. These cells are primarily responsible for cellular
immunity and delayed-type hypersensitivity responses (transplantation
immunity and immunity to select viral, bacterial, and parasitic antigens). - Bone marrow-derived or B cells. These lymphocytes are primarily respon
sible for humoral immunity and the production of circulating antibody;
under the influence of antigen, B cells can differentiate into plasma cells.
Both T and B cells can be further subdivided on the basis of cell surface
markers and specific immune functions (Table 5.1).11 Of particular impor
tance, in the context of this chapter, is the observation that there are a
number of subsets of T cells that serve to modulate the effects of other T, as