Question

Which of the following is an example of cross-presentation?

1) An endogenous peptide being presented on MHC class I
2) An exogenous peptide being presented on MHC class I
3) Recognition of MHC class II by a cytotoxic T cell
4) An exogenous peptide being presented on MHC class II
5) A B-cell antigen being presented to a T cell

Answer 1

Cross-presentation involves dendritic cells presenting exogenous antigens on MHC class I molecules to activate cytotoxic T cells, which is typically indicative of intracellular (endogenous) antigens being presented.

Step-by-step explanation:

An exogenous peptide is being presented on MHC class I.

In the immune system, antigen presentation is a vital process where immune cells such as macrophages, dendritic cells, and B cells capture antigens and present them on MHC (Major Histocompatibility Complex) molecules. Normally, endogenous antigens, such as those from viruses that replicate inside cells, are presented on MHC class I to cytotoxic T cells. In contrast, exogenous antigens from pathogens outside cells are generally presented on MHC class II to helper T cells. However, in cross-presentation, a unique mechanism allows dendritic cells to capture exogenous antigens and present them on MHC class I, thus activating cytotoxic T cells even without direct infection of the presenting cell. This is crucial for immune responses against some pathogens and in cancer immunity.