Antigen Processing and Presentation

Question 1

Intracellular antigens

Answer 1

Antigens that are located within the cytosol of a cell. Native structural proteins and enzymes as well as foreign viral proteins are examples. Intracellular antigens are processed and presented to T cells in the context of MHC class I molecules.

Question 2

Extracellular antigens

Answer 2

Antigens that are located in the extracellular environment such as bacteria and other foreign proteins. Typically, in order for these antigens to elicit an immune response, they must first be phagocytosed by an antigen presenting cell, processed, and then presented to Th cells in the context of MHC II molecules. A variant of this process is cross-presentation where extracellular antigens are phagocytosed, processed, and then presented to CD8+ T cells on a MHC class I molecule.

Question 3

Lysosome

Answer 3

A membrane-bound organelle abundant in phagocytic cells containing digestive enzymes that assists in the degradation of both extraceullar and intracellular proteins, nucleic acids, and carbohydrates. Lysosomes play a critical role in the MHC class II pathway or antigen processing.

Question 4

Invariant chain

Answer 4

A protein that binds to newly synthesized class II MHC molecules in the endoplasmic reticulum, thus preventing the binding cleft on the MHC molecule from being mistakenly loaded with native proteins present in the ER.

Question 5

Proteasome

Answer 5

A large multiprotein enzyme complex found in the cytoplasm of most cells that catabolizes proteins and peptides.

Question 6

Antigen cross presentation

Answer 6

Dendritic cells, and to a lesser extent, macrophages phagocytose extracellular antigens, process them, and present them to T cells on MHC class I molecules. Extracellular antigens are typically presented on MHC class II molecules, whereas intracellular antigens are typically presented on MHC class I molecules. Cross presentation allows for extracellular antigens to be presented to CD8+ Tc cells.