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Dendritic cells are mainly found in tissue that has contact with the outside environment such as the skin and the lining of the nose, lungs, stomach and intestines. The cells are also found in an immature state in the blood. During certain stages of development, dendritic cells develop branched projections called “dendrites,” which is why they are so named.
Once activated, dendritic cells move towards the lymphoid tissues to interact with T cells and B cells and initiate the adaptive immune response.
Dendritic cells are formed from hematopoietic progenitor cells in the bone marrow. Initially, the progenitor cells form immature dendritic cells that have endocytic capability but a low capacity to stimulate T cells. These immature cells monitor their environment for invaders such as bacteria and viruses, which they achieve through pattern recognition receptors (PRRs) such as the toll-like receptors (TLRs). As soon as these immature cells come into contact with a presented antigen, they mature and move towards the lymph nodes.
The immature dendritic cell engulfs the pathogen and breaks down its proteins, which it presents on its surface using MHC molecules once it matures. At the same time, the cell-surface receptors that act as co-receptors in T-cell activation are upregulated by dendritic cells.
Furthermore, the CCR7 receptor is also upregulated. This chemotactic receptor induces the movement of dendritic cells through the blood and towards the spleen or lymph node where they activate T cells and B cells through antigen presentation.
Activated macrophages (another type of antigen presenting cells) are known to have a lifespan of a few days, with some research suggesting this extends to weeks, and the lifespan of activated dendritic cells is understood to be similar. However, it appears that immature dendritic cells can remain in their inactivated state for significantly longer periods of time.