– MHC class I and II are two types of molecules that are encoded by the major histocompatibility complex (MHC) genes and are involved in the presentation of antigens to the immune system.

– They have some similarities and some differences, as follows:

Similarities:

1). Both MHC class I and II are composed of two chains that form a heterodimer, which is a molecule that consists of two different subunits.

2). Both MHC class I and II have a groove that binds the antigenic peptide, which is a short fragment of a protein that can be recognized by the immune system.

3). Both MHC class I and II interact with T cells, which are a type of lymphocyte that can kill infected cells or activate other immune cells.

Differences:

1). MHC class I molecules are composed of a heavy chain (alpha chain) and a light chain (beta-2 microglobulin), while MHC class II molecules are composed of two chains (alpha chain and beta chain) that are both encoded by the MHC genes.

2). MHC class I molecules present endogenous antigens, which are antigens that originate from within the cell, such as viral proteins, tumor antigens, or self-proteins that are altered by mutations or modifications.

3). MHC class II molecules present exogenous antigens, which are antigens that originate from outside the cell, such as bacterial proteins, parasitic proteins, or allergens.

4). MHC class I molecules are expressed on the surface of all nucleated cells, while MHC class II molecules are expressed on the surface of professional antigen-presenting cells (APCs), such as macrophages, dendritic cells, and B cells. MHC class I molecules interact with the CD8 molecule on cytotoxic T cells, which are the T cells that can kill infected or abnormal cells. MHC class II molecules interact with the CD4 molecule on helper T cells, which are the T cells that can secrete cytokines and stimulate other immune cells.

FUNCTIONS OF MHC CLASS III

– MHC class III is a group of genes that are located between MHC class I and II genes on chromosome 6 in humans. They encode proteins that are not involved in antigen presentation, but have other immune functions, such as complement activation, inflammation, and cytokine production. Some of the proteins encoded by MHC class III genes are:

1). Complement components C2, C4, and factor B, which are part of the classical and alternative pathways of complement activation. Complement is a system of proteins that can mark pathogens for phagocytosis, form pores in their membranes, and recruit inflammatory cells.

2). Cytokines such as tumor necrosis factor alpha (TNF-α), lymphotoxin alpha (LTA), and lymphotoxin beta (LTB), which are involved in inflammation, cell death, and lymphoid organ development. TNF-α is a pro-inflammatory cytokine that can induce fever, shock, and apoptosis. LTA and LTB are members of the TNF superfamily that can regulate the formation and function of lymph nodes and spleen.

3). Heat shock proteins such as HSP70 and HSP90, which are involved in protein folding, transport, and degradation. Heat shock proteins can also act as chaperones for antigen presentation, and as danger signals for the innate immune system.

– MHC class III genes are highly polymorphic, which means that they have many different variants in the population. This diversity may affect the susceptibility or resistance to certain diseases, such as autoimmune diseases, infectious diseases, and cancers. For example, some variants of C4 and TNF-α have been associated with increased risk of systemic lupus erythematosus, a chronic autoimmune disease that affects multiple organs.

Could you possibly be wondering.....

What are the diseases associated with MHC class III genes?

– Yes, there are some diseases associated with MHC class III genes. Some of the diseases that are associated with MHC class III genes are:

🔅 SYSTEMIC LUPUS ERYTHEMATOSUS

– SLE is a chronic autoimmune disease that affects multiple organs and tissues, such as the skin, joints, kidneys, blood, and brain. It is characterized by the production of autoantibodies that attack the body's own cells and tissues, causing inflammation and damage. Some variants of the MHC class III genes for complement components C2, C4, and factor B have been linked to increased risk of developing SLE, as they may affect the regulation and activation of the complement system, which is part of the innate immune system that can mark pathogens for phagocytosis, form pores in their membranes, and recruit inflammatory cells.

🔅 RHEUMATOID ARTHRITIS (RA)

– RA is a chronic inflammatory disease that affects the joints, causing pain, swelling, stiffness, and deformity. It is caused by the abnormal activation of the adaptive immune system, which produces autoantibodies and inflammatory cytokines that target the synovial membrane, the tissue that lines the joint cavity. Some variants of the MHC class III genes for tumor necrosis factor alpha (TNF-α), lymphotoxin alpha (LTA), and lymphotoxin beta (LTB) have been associated with increased risk of developing RA, as they may influence the expression and function of these cytokines, which are involved in inflammation, cell death, and lymphoid organ development.

🔅 COMMON VARIABLE IMMUNODEFICIENCY (CVID) AND IgA DEFICIENCY

– CVID and IgA deficiency are two types of primary immunodeficiency diseases, which are disorders that impair the function of the immune system, making the individual more susceptible to infections and other complications. CVID is characterized by low levels of immunoglobulins in the blood and impaired antibody responses to antigens. IgA deficiency is characterized by low or absent levels of immunoglobulin A (IgA), which is the main antibody found in mucosal secretions, such as saliva, tears, and breast milk. Some variants of the MHC class III genes for complement components C2, C4, and factor B have been associated with increased risk of developing CVID and IgA deficiency, as they may affect the maturation and activation of B cells, which are the immune cells that produce antibodies.

– These are some of the examples of diseases that are associated with MHC class III genes, but there may be more that are not yet discovered or confirmed. The MHC class III genes are involved in various immune functions, and their variants may influence the susceptibility or resistance to different diseases, depending on the interaction with other genetic and environmental factors.