Engulfed antigens and antigens generated within cells undergo different processing and presentation pathways.
Localisation and processing of engulfed antigens
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Intravenously introduced antigens rapidly localise in the spleen, liver, bone marrow, kidneys and lungs.
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Antigen-presenting cells engulf antigen by endocytosis. The endosome then fuses with a lysosome, where antigen is degraded into fragments such as short peptides.
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These peptides are displayed at the cell surface within class II histocompatibility molecules and may be recognised by CD4+ T cells.
Processing of antigens generated within cells
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Antigens generated within a cell, such as viral proteins in infected cells, are fragmented through processing involving cytosolic proteasomes.
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The fragments are displayed at the cell surface on class I histocompatibility molecules and recognised by CD8+ T cells.