APCs are antigen presenting cells, which display epitope proteins – exogenous antigen or fragmented antigen from phagocytosed cells – on their surfaces.

APC types : B cells : BCRs : CD1 family : CD1 proteins : CD4+ : class II MHC : dendritic cells : endocytosis : exocytosis : fragmented antigen peptides : histocompatibility molecules : γδ T cells : intact antigen : lipid antigen : macrophages : mycobacterial cell wall components : peptide antigen : phagocytic presenting cells : T cells and fragmented peptides, T cells and lipid antigens

Antigen presenting cells include:
phagocytic cells – dendritic cells, macrophages
B cells (B lymphocytes)
γδ T cells

Fragmented antigen – APCs engulf the antigen through endocytosis, then the endosome fuses with a lysosome where the antigen is digested into fragments such as short peptides. Following, exocytosis, a class II histocompatibility molecule holds the fragmented antigenic peptides at the surface of the cell, where they may be recognized by CD4+ T cells.

Intact antigen – dendritic cells can present intact antigen to B cells (not fragmented in lysosomes) by presenting the antigen on the cell surface. This antigen can bind to BCRs of the appropriate specificity, and can stimulate the B cells.

Presentation of peptide antigens for activation of naïve T cells does not reside solely in dendritic cells. A population of γδ T cells can efficiently present peptide antigens to αβT cells, and γδ T cells of the major tissue subset recognize self and foreign nonpeptide, lipid antigens presented by CD1 proteins. γδ T cells carry TCRs encoded by different gene segments than those of αβ T cells.

CD1 proteins are a family (CD1a-e) of cluster of differentiation glycoproteins related to the class I MHC molecules. CD1 are involved in the presentation of lipid and glycolipid antigens, particularly self, microbial, and mycobacterial cell wall components, to CD1-specific T cells.

The human CD1 family of transmembrane glycoproteins are encoded by five CD1 family genes organized in a cluster on chromosome 1. CD1 glycoproteins form heterodimers with beta-2-microglobulin. CD1 family members are considered to differ in cellular localization and specificity for particular lipid ligands. The CD1a protein (R4, T6, CD1, FCB6, HTA1) localizes to the plasma membrane and to recycling vesicles of the early endocytic system. Alternatively spliced transcript variants have been observed.[e]

ф activationalternative splicing ф antibodies ф antigen ф B cells cell membranes ф costimulation ф dendritic cellsendosomesexosome ф helper T cell ф killer T cellslysosome ф macrophages ф MHC ф pathogens ф pattern-recognition receptors ф phagocytephagocytosisreceptor-mediated endocytosis ф surface receptors ф γδ T cells ф T cells

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