Page 275 - Veterinary Immunology, 10th Edition

VetBooks.ir Dendritic Cells

Dendritic cells are specialized antigen-presenting cells that can
initiate adaptive immune responses. They are located at sites where

they are most likely to encounter foreign antigens, that is, in
lymphoid organs and under epithelial surfaces. DCs perform three
major functions. First, they serve as sentinel cells and activate
innate defenses when they first encounter invaders. Second, they
process exogenous antigens and thus initiate adaptive immune

responses. Third, they regulate adaptive immunity by determining
whether an antigen will trigger an antibody-mediated or a cell-
mediated response, or even prevent an immune response (a

condition called tolerance). DCs are at least 100 times more effective
antigen-presenting cells than macrophages or B cells. DCs can take
up many different antigens, including dead microorganisms,
soluble foreign antigens in tissue fluids, and antigens released by
dying cells, and present them to T cells. DCs are the only antigen-

processing cells that can activate T cells that have never previously
encountered an antigen (naïve cells) and therefore are essential for
initiating primary immune responses.

Origin

All DCs are derived from hematopoietic stem cells in the bone
marrow. Immature DCs migrate through the body and form
networks in virtually every tissue. Monocytes may also develop

into classical DCs when exposed to appropriate cytokines. DCs are
found in all organs except the brain, parts of the eye, and the testes.
They are especially prominent in lymph nodes, skin, and mucosal
surfaces—sites where invading microbes are most likely to be

encountered.

Structure

The shape of a DC depends on its state of activation. Typically,
however, they are characterized by having a small cell body, about

13-15 µm in diameter with many long cytoplasmic processes known

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