Presenting Endogenous and Exogenous Antigens

The terms “endogenous antigens” and “exogenous antigens” refer to the origin of the antigens in relation to the cell presenting them.

1. Endogenous antigens:
   • These antigens originate from within the cell itself. They are typically derived from proteins synthesized by the cell’s own machinery.
   • Endogenous antigens are loaded onto Major Histocompatibility Complex (MHC) Class I molecules.
   • Presentation of endogenous antigens occurs primarily to cytotoxic T lymphocytes (CD8+ T cells).
   • MHC Class I molecules present endogenous antigens to CD8+ T cells to elicit immune responses against infected or abnormal cells. These antigens can include peptides derived from viral proteins synthesized within infected cells, abnormal proteins from cancerous cells, or proteins produced by intracellular bacteria.
2. Exogenous antigens:
   • These antigens originate from outside the cell. They are typically derived from proteins that the cell has taken up from the extracellular environment through processes like phagocytosis or endocytosis.
   • Exogenous antigens are loaded onto Major Histocompatibility Complex (MHC) Class II molecules.
   • Presentation of exogenous antigens primarily occurs to helper T lymphocytes (CD4+ T cells).
   • MHC Class II molecules present exogenous antigens to CD4+ T cells, which can then stimulate other immune cells or coordinate immune responses. These antigens can include peptides derived from engulfed pathogens, such as bacteria or fungi, as well as proteins from other sources present in the extracellular environment.

Next Topic: Details of MHC I Antigen Processing and Presentation

Source: ChatGPT response prompted and edited by Joel Graff.

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