Structural Biochemistry, Protein function, Major Histocompatibility Complex (MHC), MHC Class II

The Class II MHC molecules are made by antigen presenting cells (dendritic cells, macrophages, and B cells) which help trigger an acquired immune response. Antigen presenting cells ingest foreign antigens via phagocytosis or endocytosis and break it down into peptide fragments that bind to MHC Class II molecules which bring the antigen fragment to the surface of the cell to be recognized by the T cell receptor on the surface of Helper T Cells. The T cell receptor binds to the MHC II-antigen fragment complex with the aid of the CD4, a protein receptor on the surface of helper T cells, and once bound, the helper T cell becomes active and begins proliferating and secreting cytokines.

When Helper T cells proliferate, both memory helper T cells, and to a larger extent, active helper T cells are made. Memory helper T cells contribute to the acquired immune response by quickly proliferating in the case of a secondary "attack" by the antigen producing a faster, stronger response to the antigen. Active helper T cells have a shorter life span than memory helper T cells and help initiate an immune defense against the pathogen by secreting cytokines that stimulate the activation of nearby B cells and Cytotoxic T cells. Once activated, cytotoxic T cells destroy infected cells via apoptosis while the plasma B cells secrete specific antibodies that neutralize and eliminate the pathogen. The cytotoxic T cells and B cells will also proliferate to produce their respective memory cells for use in a second attack by the pathogen.

MHC CLASS II

(Image: A09231315, CC BY-SA 3.0)

In the case of HIV, the Helper T cells are the cells that are directly attacked by the virus. They are completely killed off, which means that no more Helper T cells are able to create cytokines and call the other phagocytic cells to reproduce and create an immune response. This causes the death of all Helper T cells, as well as the death of the immune system. Because of this, the person that has HIV/AIDS does not die of the virus, but can die of (i.e.) a common cold, because the immune system has no way to respond to it.

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