Let's now turn to dendritic cells.
There are two fundamentally different types of
cell that are referred to as dendritic cells.
Cells that are just called dendritic cells and other
cells that are called follicular dendritic cells.
And really the only thing that these two cell
types have in common is their morphology.
They're named because they have long processes
that rather resemble nerve dendrites.
That's where their name comes from.
But apart from their appearance, they
really have very little in common.
Although, actually they are both involved
in showing antigen to lymphocytes.
So, they do have that in common.
So, what are their differences?
Well the dendritic cells-- and if you ever come across
the term dendritic cell, this is the cell that is being
referred to, are like the vast majority of cells of the
immune response derived from hematopoietic stem cells.
In contrast, this is pretty much the only cell
of the immune response that isn't derived from
hematopoietic stem cells; the follicular dendritic
cells are derived from mesenchymal stem cells.
Dendritic cells are present throughout the body
whereas the follicular dendritic cells are pretty
much restricted to structures called germinal centers
which you find in secondary lymphoid tissues.
Dendritic cells are
When they're present in the tissues they
can engulf microorganisms and fragments
of microorganisms, whereas follicular
dendritic cells are never phagocytic.
The dendritic cell possesses on its cell surface
as well as many other molecules, MHC class II
molecules and a variety of co-stimulatory molecules,
particularly the B7 co-stimulatory molecules.
Whereas follicular dendritic cells
lack MHC class II and lack B7 molecules.
The dendritic cells can
activate helper T-cells.
That's really the job of dendritic
cells, is to activate helper T-cells,
particularly na've helper T-cells that
have never encountered antigen before.
Seems that for that particular job, activating a na've T-cell,
dendritic cells are absolutely
obligatory, whereas follicular dendritic
cells do not activate helper T-cells, but instead they're
specialized to show antigen in its
natural native form to B-cells.
There are three cell types of the innate response that are
particularly specialized for producing inflammatory mediators.
Eosinophils which are present both in the blood
and in the tissues, and characteristically
as well as many, many other molecules they
secrete, produce a molecule called major basic protein.
And this they release to the
immediate environment of the cell.
Basophils which are present in the blood and
produce histamine, prostaglandins and leukotrienes.
And mast cells which are very closely related
to basophils are present in mucosal surfaces
and in connective tissue, and they also release
histamine, prostaglandins and leukotrienes.
Turning now to innate lymphoid cells,
perhaps the best characterized of
several different types of innate
lymphoid cell is the Natural Killer cell.
And these can induce apoptotic cell death
in infected cells using one of two different
pathways: the Fas/FasLigand pathway, or
alternatively the Granzyme/Perforin pathway.
Other types of Innate Lymphoid Cells
have functions rather similar to the
helper T-cells and regulatory T-cells
of the adaptive immune response.