ABSTRACT
The immune system is a complex set of cellular and soluble mediators that protects the
body against foreign substances, including infectious agents and certain tumor cells.
Immune cells are located throughout the body, either in discrete organs, such as the spleen,
thymus, and lymph nodes, or in diffuse accumulations of lymphoid and myeloid cells, as
are found in association with the skin, lung, and gastrointestinal tract-strategic locations
for detection of entering pathogens and exogenous proteins. Protection may rely on
responses to proteins or carbohydrates that are unique to a particular pathogen or cell
(antigens) or to components of organisms that are widely shared (e.g., viral double
stranded RNA, components of bacterial cell walls). Antigen-driven responses are referred
to as antigen-specific or adaptive, whereas responses to shared components are referred to
as nonspecific or innate. Adaptive responses require recognition of foreign antigens via
complex interaction of cell surface molecules, production of growth factors, lymphocyte
proliferation (clonal expansion), and implementation of effector mechanisms that
ultimately mediate destruction of the foreign threat. The response is very specific but is
rather slow (generally 3-7 days to the peak response). In contrast, innate responses do
not require antigen recognition or clonal expansion and thus provide a rapid response
(!24 hours) to infection. Innate responses are well conserved phylogenetically; the same or very similar responses that mediate resistance to infection in invertebrates act as a first
line of defense in mammals to destroy similar classifications of pathogens. In mammals,
nonspecific effector cells include macrophages (production of proinflammatory mediators,
phagocytosis of pathogens and dead cells), natural killer (NK) cells (contact-dependent
killing of certain tumor cells and certain pathogens), and neutrophilic leukocytes
(phagocytosis of bacteria). A variety of soluble mediators are important to innate
responses as well, including complement (through lysis of cells and augmented
phagocytosis of bacteria) and cytokines (through modulation of the inflammatory
response).
