Question

You identify a new non-envelope virus. It is found to generate a potent antibody response in humans to its capsid. Why is the capsid epitope recognized by the B-cell not necessarily or likely to be the antigenic peptide recognized by one of the MHCIIs it expresses?

Answer 1

The immune system has developed methods to fight against endogenous and exogenous antigens. As we know that, viruses are pathogens that propagate only when they enter the inside of the cell they are processed through endogenous antigen pathway. Here the viral proteins are recognized and are processed by proteasomic digestion and are presented to type MHC-I class of antigen presenting complex. Thus presented antigens (on MHC-I) are recognized by cytotoxic T cells (CD8+ T-Cell) which destroy the cell presenting the antigen. Exogenous antigens are processed by antigen presenting cells (example: macrophages) and presented to MHC-I cells which will be interacted by CD4+ T- cells.

The B cells have antibodies on their surface and when these antibodies bind to capsid proteins of virus, the particular B cell propagate and become antibody secreting plasma cell which secrete antiobody specific to the capsid of virus, thus neutralizing the virus and also activating other branches of immune system.

MHC I and II only recognize the processed antigenic peptides but not the bulky soluble antigens. also antigen recognition is different in antibodies and the MHC complexes. The epitopes recognized by the antibodies are usually located on the surface of the target proteins and it is less probable that antibodies recognize epitopes hidden inside the proteins. In case of MHC-II it binds to and present the entigenic peptides or epitopes which are hidden inside the protein and exposed due to digestion process of the antigen processing pathway.

Thus because of difference in their binding pattern and antigen recognition it is less likely that epitope recognized by the B-cell (antibody) to be the antigenic peptide recognized by one of the MHCII it expresses.