The Biology Project: Immunology

Immunology Problem Set

Problem 10: Regulation by the MHC

Tutorial to help you answer the question:

The significance of the major histocompatibility complex (MHC) in the immune response:

A. Serves to minimize autoimmunity or "self-reactivity" of the immune system
B. Serves to present fragments of antigens to T-cells.
C. Used by helper T-cells to regulate the expansion of antibody producing B-cells.
D. All of the above.


The major histocompatibility complex (MHC) is a series of genes that code for cell surface proteins which control the adaptive immune response. The system is called H2 in mice and HLA (human lymphocyte antigen) in humans. Class I MHC contains three genes called HLA-A, B, and C; proteins from these genes are expressed on almost all cells. Class II MHC genes are called HLA-DR, DQ, and DP; their proteins are expressed on antigen-presenting macrophages, dendritic cells and B cells.

The function of these proteins is to present fragments of antigens to T cells. The receptor of T cells can only recognize antigen fragments in complex with MHC proteins.

Following phagocytosis of a pathogen, fragments of the pathogen are complexed with MHC proteins and displayed on the surface of the macrophage or dendritic cells of the innate immune system. If the cell encounters a dangerous pathogen, a co-receptor called B7 is produced. This is a crucial step for turning on the adaptive immune response and developing a memory of pathogen threats. Those helper T cells with a receptor that recognized the antigen fragment of the MHC complex can be stimulated to proliferate and be activated if the co-receptor is also displayed.

The dendritic cell of the innate system is used to initiate adaptive immunity.

Following activation by an antigen presenting cell, the Helper T cell signals B cells and cytotoxic T cells to launch an immune response. This activation of helper T cells occurs in the lymph nodes. Following activation is another clonal selection, and helper T cells with the correct receptor are activated and proliferate.

Additional detail of MHC molecule/antigen binding

The space between the helices of this class I MHC molecule (HLA-A2) can bind peptides inside the cell and carry them to the cell surface. This is the form recognized by the T cell receptor on cytotoxic T cells. Class II have similar structures, but are specific for presenting antigen fragment to helper T cells.

Class I MHC molecule complexed with a peptide from HIV. A helper T cell would recognize this complex on the surface of an antigen presenting cell before a cytotoxic T cell destroys this HIV infected cell.


Problem 10 | Answer | Problem 11

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