Immunology Questions and Answers – Exogenous Pathway of Antigen Processing and Presentation

This set of Immunology Multiple Choice Questions & Answers (MCQs) focuses on “Exogenous Pathway of Antigen Processing and Presentation”.

1. Where are exogenous proteins processed?
a) Cytosol
b) Endosomes
c) Cytoplasm
d) Lysosomes
View Answer

Answer: b
Explanation: Exogenous proteins are processed in endosomes. These proteins act as mediator in order to enhance the exogenous pathway of processing and presentation of antigens. The main reason of exogenous proteins to be processed in endosomes is because endosomes regulate the trafficking of these proteins among the subcellular compartments of the exogenous pathway. Endosomes are membrane-bound vesicles found in the cytoplasm where proteins get processed via endocytosis.

2. Which of the following types of antigens are NOT involved in carrying out the exogenous pathway of antigens?
a) Viral antigens
b) Self-antigens
c) Bacterial antigens
d) Fungal antigens
View Answer

Answer: b
Explanation: Antigens involved in carrying out exogenous pathway of their processing and presentation include Viral, Bacterial, Fungal as well as Protozoal and Parasitic antigens. These antigens are also called as exogenous antigens as their major role is in regulating the exogenous pathway. These antigens are derived from exogenous proteins processed outside the cell. These antigens are different from the endogenous antigens as they act as foreign antigens that enter the body from outside either by ingestion or inhalation.

3. Exogenous antigens are associated with which molecules in order to activate cells?
a) Class II MHC
b) Peptide molecules
c) Class I MHC
d) Antigen presenting cells
View Answer

Answer: a
Explanation: Exogenous antigens are associated with MHC class II molecules and play a very important role in activating helper CD4+ T cells. This activation provides great help in regulating the B and cytotoxic T cells. MHC class II molecules are very important as they bind antigenic peptides which are produced in endosomes. This binding forms antigen peptide-MHC class II complex which results in limiting the complete hydrolysis of proteins. This complex in turn activates the naïve CD4+ T cells.

4. Exogenous antigens are processed and presented by which cells?
a) T cells
b) B cells
c) CD4+ T cells
d) Antigen presenting cells
View Answer

Answer: d
Explanation: Exogenous antigens which are associated with MHC class II molecules activate the helper CD4+ T cells but the entire processing and presentation pathway is carried out by the Antigen Presenting Cells. These cells are also designated as APCs. APCs internalize antigens by phagocytosis or endocytosis. APCs are very important for adaptive immune responses as the functioning of helper T cells is totally dependent on APCs. This means that even if antigen-peptide-MHC class II complex activates the helper T cells, they won’t be regulated unless they come in contact with APCs.

5. Which of the following molecule binds to Class II MHC molecules to prevent the activity of endogenous peptides?
a) Helper CD4+ cells
b) Cytosolic proteases
c) Calnexin
d) Invariant Chain
View Answer

Answer: d
Explanation: In some rare cases, endogenous peptides enforce their activity in order to bind to the MHC class II molecule to form a complex. In such cases, a protein named Invariant chain plays a very important role as it binds to Class II MHC molecules to prevent the binding of endogenous peptides. Class II MHC molecule structurally consist of a groove. The invariant chain temporarily occupies this groove till the exogeneous peptides are not successfully transported to bind to Class II MHC.

6. Dendritic cells are also known as professional APCs as they have greater responsibility than B cells.
a) True
b) False
View Answer

Answer: a
Explanation: Dendritic cells are also known as professional APCs because they work specifically in activating T cells. Dendritic cells deliver signals in order to activate T cells. They are most efficient to function in primary immune response. On the other hand, B cells are specific for a given antigen and do not spontaneously react to primary immune response. They are efficient to function in secondary responses only.

7. Which is the correct order of activities showcased by exogenous pathway?
i) Generation of MHC class II molecules
ii) MHC-peptide transport
iii) Class II MHC peptide loading
iv) Generation of Peptide from proteolysis
a) i—ii—iii—iv
b) iv—i—iii—ii
c) ii—iii—i—iv
d) i—iii—iv—ii
View Answer

Answer: b
Explanation: Processing and presentation of antigens require 4 major activities. Firstly, there is generation of peptides in the endosomes which is carried out by proteases to degrade antigens into peptide fragments. Meanwhile, MHC class II molecules are generated in other vesicles with the help of a protein named invariant chain. Class II MHC molecules along with invariant chain are then transported to endosomes where processed peptides are present. The peptides are then loaded on to the MHC class II molecules and are transported into membrane bound vesicles to the plasma membrane.

8. Which molecule is responsible for stabilizing empty MHC class II molecules?
c) H-2M
View Answer

Answer: a
Explanation: HLA-DM is a non-classical Class II like MHC molecule which is responsible to bind to empty MHC class II molecule in order to stabilize it. It helps in the release of CLIP fragment for the completion of binding mechanism performed by antigens. On the other hand, MHC-DM is a molecule expressed only in endocytic vesicles. If it is present in the vicinity of HLA-DM, it usually inhibits the activity of HLA-DM.

9. Which molecule is left in the binding groove of MHC II molecule in the endocytic compartments?
a) Heat shock proteins
c) Tapasin
View Answer

Answer: b
Explanation: Invariant chain binds to the groove of MHC II molecule in endoplasmic reticulum in order to prevent exogenous peptide binding. The invariant chain also transports the MHC II molecule from golgi apparatus to the endocytic compartments. In these compartments, invariant chain is cleaved. A part of this chain remains in the groove of MHC II molecule. This part is known as CLIP (Class II associated Invariant chain Peptide).

10. What is the length of peptide fragments that are loaded to the MHC class II molecules?
a) 5-10 amino acids long
b) 10-12 amino acids long
c) 13-18 amino acids long
d) 20-25 amino acids long
View Answer

Answer: c
Explanation: MHC class II proteins usually have a length of peptide fragments of 13-25. Endocytic vesicles are acidified which result in activation of proteases. This results in degradation of proteins into fragments. The endocytic vesicles have low pH which means they are acidic and they have more than 40 hydrolases which cuts the antigen into peptides 13-18 amino acids and this length encumbered on MHC II molecules.

Sanfoundry Global Education & Learning Series – Immunology.

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