Question

During endogenous antigen processing, the antigen is cleaved in which of the following organelles?
A. Endoplasmic reticulum
B. Proteasomes
C. Golgi bodies
D. Lysosomes

Answer 1

Hint: Endogenous antigens are those produced within previously healthy cells as a result of normal cell metabolism or viral or intracellular bacterial infection (which both change cells from the inside in order to reproduce). Antigens that enter the body from the outside, such as bacteria, fungi, protozoa, and free viruses, are known as exogenous antigens. The primary distinction between exogenous and endogenous antigens is that exogenous antigens enter the body from the outside, whereas endogenous antigens are produced within the body. Furthermore, when infections infect cells, they develop endogenous antigens.

Complete answer:
Option A: TAP (transporter associated with antigen processing) is a protein that traverses the rough endoplasmic reticulum membrane and carries peptides into the rough endoplasmic reticulum lumen.
So, option A is not correct.
Option B: On the cell surface of MHC class I molecules, the endogenous route is used to present cellular peptide fragments broken down by proteasomes. The antigen processing transporter spans the rough endoplasmic reticulum membrane and carries peptides into the lumen of the rough endoplasmic reticulum. The peptide attaches to the cleft of the MHC class I molecule after it enters the ER lumen, stabilising it and allowing it to be transported to the cell surface by the Golgi apparatus.
So, option B is correct.
Option C: The endogenous pathway is utilised to provide cellular peptide fragments broken down by proteasomes on the cell surface of MHC class I molecules. The antigen processing transporter crosses the rough endoplasmic reticulum membrane and transports peptides into the reticulum lumen. After entering the ER lumen, the peptide binds to the cleft of the MHC class I molecule, stabilising it.
So, option C is incorrect.
Option D: When foreign organisms are phagocytosed, an endosome is formed, which is joined with lysosomes, which contain enzymes that destroy and break down the organisms.
So, option D is not correct.

Therefore, Option B is the correct answer.

Note:
On MHC class I molecules, the endogenous route is used to deliver cellular peptide fragments on the cell surface. If the cell had been infected by a virus, viral peptides would be delivered to the immune system, allowing it to recognise and kill the infected cell. Ubiquitination marks worn-out proteins in the cell, indicating that they should be degraded by the proteasome.
Proteins are broken down into peptides by proteasomes, some of which are nine amino acids long (suitable for fitting within the peptide binding cleft of MHC class I molecules). TAP (transporter associated with antigen processing) is a protein that traverses the rough endoplasmic reticulum membrane and carries peptides into the rough endoplasmic reticulum lumen (ER).