Page 278 - Veterinary Immunology, 10th Edition

stimulates antibody-mediated immune responses designed to
VetBooks.ir protect animals against extracellular invaders. Which of these

helper cell types is activated depends on the DCs.

Classical DCs

The relationships between macrophages and dendritic cells is
unclear and may differ between species. Blood monocytes are the
immediate precursors of some tissue macrophages and some cDCs.
Which cell type is produced depends on the mixture of cytokines
and cells encountered by the monocyte as it differentiates. Each

type can convert to the other until late in the differentiation process.
cDCs can therefore be considered part of the mononuclear
phagocytic system being derived from a common stem cell,

respond to the same growth factors, express the same surface
markers, and in effect are in no specific way uniquely different from
other macrophages. Their main function is antigen presentation and
induction of T cell responses.

Plasmacytoid DCs

Plasmacytoid DCs (pDCs) are long-lived cells found in blood, bone
marrow, and lymphoid organs. They lack myeloid antigens but
retain some lymphoid characteristics. pDCs serve as an early
warning system for viral infections since they are rapidly activated

by viral nucleic acids. They respond to viruses by synthesizing
massive amounts of the type I interferons (IFN-α and IFN-β). (Ten
to 100-fold more than other cell types.) They are also major
producers of type III interferons (IFN-λ)(Chapter 27). Their

numbers increase during infection. pDCs have a unique ability to
link innate and adaptive immunity. After producing large amounts
of type I interferon, they are still able to differentiate into mature
DCs that can stimulate naïve T cells. Because pDCs secrete large

amounts of IFN-α, they also activate natural killer (NK) cells
(Chapter 19).

Langerhans Cells

Langerhans cells are specialized, long-lived antigen-presenting cells
found in the skin. They contain characteristic rod- or racquet-

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