Question

1)The following are the steps for B-cell activation. The steps are in an incorrect order.

1. B cells display antigens in MHC class II receptors on the cell surface.

2. Antibodies released by plasma cells bind to antigen so they will be destroyed by other cells of the immune system.

3. B cells rearrange their DNA to create a unique B-cell receptor.

4. B cells become activated by interacting with helper T cells.

5. B cells undergo clonal expansion.

6. B cells digest antigens that bind to the antibodies on their surface.

What are the steps in their correct order of occurrence?

	a.	3, 6, 1, 4, 5, 2
	b.	1, 6, 3, 4, 2, 5
	c.	3, 4, 5, 6, 1, 2
	d.	6, 3, 1, 2, 5, 4

2) How are antigens processed before binding to MHC class I molecules?

a-  They are digested in the cytoplasm of the cell by the proteosome.

b-  They are digested by enzymes in the endoplasmic reticulum

c-They are digested by enzymes in the late endosomal compartment

d- They are digested by enzymes in the endocytic vesicle.

3)  Both MHC class I receptors and MHC class II receptors are synthesized on the rough ER. What keeps MHC class II receptors from binding to antigens while in the ER?

a- The MHC class II receptor has its binding site covered by a protein called the invariant chain. Until that is removed, it is unable to bind to any antigens.

b- There are no antigens in the ER compartment for MHC class II to bind.

c- The MHC class II molecules are kept in a separate compartment of the ER so that they cannot mix with antigens yet.

d- Any antigens bound to the MHC class II molecule are removed by enzymes in the Golgi complex.

Answer 1

Question 1:-

In B cell activation, the first step is the expression of B cell receptor as it helps in capturing the antigen by first interacting with it. So first by genes rearrangement, a unique B cell receptor is synthesized.

Then second step is digesting the antigen which reacted with the Receptor. This is done by Endocytosis.

After digestion of antigen, its peptide fragments are presented on MHC II molecules on the cell surface.

MHC II then interacts with T Helper cells which releases cytokines if the peptide presented is an antigen to them. These cytokines activate the B cells.

After activation, these B cells which have presented the peptide on them, goes into clonal expansion mode. Since B cells have a lot of different receptors, clonal expansion make it possible for them to divide only those B cells which are expressing the same receptor.

After clonal expansion, plasma cells (Special B cells which are capable of secreting a lot of antibodies) secrete antibodies which mark the antigen as a target for other immune cells.

So the correct order is 3, 6, 1, 4, 5, 2.

The right option is Option (a)

Question 2:-

MHC I complex present viral peptides for Cytotoxic T cells. These viral particles are first ingested and then degraded inside the cell. This degradation of viral protein is achieved by Proteasome in the cell cytosol and the resulting peptide is transported to the ER.

So the right option is Option (a) They are digested in the cytoplasm of the cell by the proteosome.

Question 3:-

While MHC II molecules are inside the ER, their peptide binding cleft is occupied and blocked by a specific glycoprotein. This glycoprotein block the site of peptide binding so that MHC 2 will not interact with antigens involved in MHC I pathway. This glycoprotein is known as Invariant chain.

So the right option is Option (a) The MHC class II receptor has its binding site covered by a protein called the invariant chain. Until that is removed, it is unable to bind to any antigens.