MHC class 1&2

Below we have discussed the structure of class 1 and class 2 MUC.

The Major Histocompatibility Complex (MHC) is called the human leukocyte antigen (HLA) complex in humans and the H-2 complex in mice. Almost all human cells contain HLA molecules on their plasma membranes, only RBC cells do not have MHC. HLA molecules can be divided into three classes;

Class 1 molecules are found on all type of nucleated body cells.

Class 2 molecules appear only on cells that can process non self materials and present antigens to other cells (i.e., macrophages, dendritic cells, and B cells).

Class 3 molecules include various selected proteins that have immune function. 

Class 1 MHC

1. Class 1 MHC molecules consist of a complex of two protein chains, one with a mass of 45,000 Daltons (Da), alpha chain, beta 2- microglobulin. 
2. The alpha chain can be divided into three functional domains, designated alpha 1, alpha 2, alpha 3. Only non- nucleated cells ( red blood cells) lack class 1 MHC  molecules.
3. Beta 2- Microglobulin:- It is associated with a non-covalently linked, which helps in stabilizing the structure but is not encoded by the MHC gene complex .
4. The alpha 1 and alpha 2 domains form the peptide binding groove, where antigens are presented. This groove accommodates peptides that are typically 8-10 amino acids long.

Function

• MHC class I molecules present peptides derived from intracellular proteins to cytotoxic T cells ( CD8+ cytotoxic T cells).
• This pathway is crucial for the detection and elimination of infected or malignant cells.
• If the CD8+ T cells recognize the presented peptide as foreign, they initiate a response to kill the infected or abnormal cell.
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Class 2 MHC

1. Both the alpha and beta chains contribute to the formation of the peptide-binding groove.
2. The alpha 1 and beta 1 domains form the peptide binding groove, which can bind longer peptides, typically 13-25 amino acids.
3. Both α and β chains span the membrane and anchor the molecule to the cell surface.
4. Cytoplasmic tails: Extend into the cell and help in signaling and transport.
5. Found only on antigen-presenting cells (APCs): dendritic cells, macrophages, and B cells.

Function

• MHC class 2 molecules present peptides from extracellular sources (eg., bacterial proteins, toxins) to helper T cells ( CD4+ cytotoxic T cells).
• This is critical for initiating immune responses such as antibody production and activating other immune cells.
• MHC class 2 molecules are primarily expressed on antigen - presenting cells (APCs) such as dendritic cells, macrophages, and B cells.
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