The term antigen presenting cell
refers to the cells that constitutively express class II MHC molecules and so
can present antigen to help T-cells.
Dendritic Cells
Dendritic cells are responsible for
launching most adaptive immune response and particularly for primary responses.
Dendritic cells arise from either the myeloid or lymphoid lineage of the bone marrow. They are found as diffuse, minor resident population in all surface
epithelia and most other solid tissues.
Their unique efficiency at
capturing, transporting and presenting antigen and attracting and activating
specific T-cells, make mature dendritic cells the most potent APCs known. Under certain conditions a single
dendritic cell can activate up to 3000 T-cells
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Macrophages
Macrophages are widely distributed
in lymphoid and non lymphoid tissues and because of their prodigious phagocytic
capacity are specially important for presenting antigen from particulate immonogens
such bacteria. Their many broad specificity receptors enable macrophages to capture
a wide range of pathogens. Macrophages are highly efficient at capturing
antibody coated antigens using their surface Fc receptors.
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B –Lymphocytes
Although B cells lack significant
phagocytic activity, they are able to capture, process, and represent some
antigen to help T cells. They are specially effective in presenting the
antigens that bind specifically to surface immunoglobulins.
Natural Killer Cells
Natural killer cells are large
granular lymphocytes that use cytoplasmic granules containing perforins to kill
the target cells. Unlike T cells, NK cells do not express a cell surface
TCR/CD3 complex. They also lack CD4, the maker for T helper cells, about half
of human NK cells express CD8, the maker for cytolytic T cells, but only one
form of CD8 is expressed.
The most important role of NK
cells appear to be in host defense against infection by intracellular agents, including
certain viruses, bacteria, and parasites.