Question

Imagine a patient unable to express MHC II, at least in their B cells. Answer the following. A). What is required to induce MHC II expression? Short answer - briefly, list steps. B) What is the role of MHC II (what does it present and to what receptor and cell type)? C) What effects would you expect B cell deficiencies in MHC II expression to have?

Answer 1

A)Major Histocompatability Complex class II molecules are heterodimeric glycoproteins composed of an alpha and beta chain;together with MHC class I play a pivotal role in the adaptive branch of immune system.

MHC class II molecules is meant to present extracellular pathogens.(ie.antigen)

First step-phagocytosis;acquire pathogen through phagocytosis-extracellular proteins are endocytosed.

Second step:pathogen is broken down and digested in lysosome.

Third step:Loading of MHC Class II molecule with the resulting epitopic peptide fragment

Fourth step:Migration of MHC Class II molecules to the surface to present the antigen to a helper T cell.

MHC II active Helper T cell helps release cytokines and and other things which help induce other cells which help to combat the pathogens outside the cell

B)MHC Class II molecules are a class of Major Histocompatability complex molecules normally found only on professional antigen presenting cells.

Main function of MHC II molecules is to present processed antigens(peptides),which are derived from exogenous sources,to CD4(+) T lympocytes.MHC class II are thereby critical for the initiation of the antigen specific immune response.

MHC Class II molecules are normally found on antigen presenting cells such as dentritic cells,mononuclear phagocytes,some endothelial cells,thymic epithelial cells and B cells.

C)In humans,MHC Class II protein complex is encoded by Human leukocyte antigen gene complex(HLA).Mutation in the HLA gene can lead to bare lymphocyte syndrome(BLS) whichis a type of MHC Class II deficiency.

This result in depletion of CD4+T cells.Deficient MHC II molecules cannot present antigen to T cells and properly activate T cells.Not only T cells,B cells are affected as the no.of T cells decreases,T cells cannot interact and activate B cells.(normally B cells activated by T cells; divide,proliferate and differentiate into plasma cells,and produces antibodies).Therefore in MHC class II deficiency the entire immune response(cell mediated-T cell as well as the antibody mediated-B cell) is affected.