Antigen-presenting cells express both molecules
Class I and Class II HLA
Professional antigen-presenting cells represent a set of cells of haematopoietic origin sharing several characteristics, the main of which is to express in the basal state Class II HLA molecules, allowing the presentation of exogenous antigens. These Class II HLA molecules are expressed in addition to Class I HLA molecules, present on all nucleated cells and platelets.
Why do all nucleated cells and platelets express Class I HLA molecules?.
All of the cells in the body are subject to permanent stress, physiological (cell division, exercise of their unctions, etc.) or not (environmental exposures, infections, etc.). One of the functions of the immune system, and in particular of CD8+ T cells, is to eliminate these cells infected or damaged by stress which have the potential to degenerate towards malignant processes. To recognise these cells, CD8+ T cells constantly "scan" the peptides embedded in HLA class I molecules. CD8+ T cells have in fact been educated to tolerate self peptides, and eliminate cells presenting peptides other than those of the self.
In addition to possessing Class II HLA molecules, professional antigen-presenting cells:
- have all the cellular machinery necessary for the capture of the antigen, its intracellular trafficking, its degradation and its processing into Class I and Class II HLA molecules (see antigen presentation)
- express costimulatory, migration, and cell adhesion molecules.
- synthesise cytokines and chemokines, which allow the recruitment of other immune actors and their polarisation if necessary
From the most specialised to the least specialised professional antigen-presenting cells, we find:
