II vs. MHC I

Attribute	II	MHC I
Function	Antigen presentation to CD4+ T cells	Antigen presentation to CD8+ T cells
Cellular Location	Expressed on antigen-presenting cells (APCs)	Expressed on most nucleated cells
Peptide Binding	Binds peptides derived from extracellular antigens	Binds peptides derived from intracellular antigens
Peptide Length	Binds peptides of variable length (usually 13-25 amino acids)	Binds peptides of fixed length (usually 8-10 amino acids)
Antigen Recognition	Recognizes antigens in the context of MHC II molecules	Recognizes antigens in the context of MHC I molecules
CD Molecule	CD4	CD8
Antigen Source	Extracellular pathogens, including bacteria and fungi	Intracellular pathogens, including viruses and tumor cells
Immune Response	Activates helper T cells and promotes humoral immune response	Activates cytotoxic T cells and promotes cell-mediated immune response

Introduction

The immune system is a complex network of cells, tissues, and molecules that work together to defend the body against pathogens and foreign substances. Two crucial components of the immune system are the major histocompatibility complex class II (MHC II) and major histocompatibility complex class I (MHC I) molecules. These molecules play essential roles in presenting antigens to immune cells, but they differ in their expression patterns, functions, and interactions with immune cells.

Expression Patterns

MHC II molecules are primarily expressed on antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells. These cells are responsible for capturing, processing, and presenting antigens to T cells. In contrast, MHC I molecules are expressed on almost all nucleated cells in the body, including epithelial cells, fibroblasts, and endothelial cells. This widespread expression allows MHC I to present antigens derived from intracellular pathogens or abnormal cellular components to cytotoxic T cells.

Structure

MHC II molecules consist of two chains, an α chain and a β chain, both encoded by genes within the MHC II region. These chains form a heterodimer that creates a peptide-binding groove. The peptide-binding groove of MHC II molecules is open at both ends, allowing for the presentation of longer peptides, typically 13-25 amino acids in length. On the other hand, MHC I molecules consist of a heavy chain and a small protein called β2-microglobulin. The peptide-binding groove of MHC I molecules is closed at one end, restricting the presentation of shorter peptides, usually 8-10 amino acids in length.

Antigen Presentation

MHC II molecules primarily present antigens derived from extracellular pathogens that have been internalized by APCs through phagocytosis or endocytosis. These antigens are processed within the endosomal compartments of the APCs, where they are broken down into smaller peptides. The peptides then bind to the peptide-binding groove of MHC II molecules, which are subsequently transported to the cell surface for presentation to CD4+ helper T cells. This interaction between MHC II and CD4+ T cells is crucial for initiating and coordinating immune responses.

On the other hand, MHC I molecules present antigens derived from intracellular pathogens or abnormal cellular components, such as viral proteins or tumor-associated antigens. These antigens are processed within the cytosol of the infected or abnormal cells by the proteasome. The resulting peptides are then transported into the endoplasmic reticulum, where they bind to the peptide-binding groove of MHC I molecules. The MHC I-peptide complex is subsequently transported to the cell surface for recognition by CD8+ cytotoxic T cells. This recognition triggers the destruction of the infected or abnormal cells.

Interactions with Immune Cells

MHC II molecules interact primarily with CD4+ helper T cells. The binding of the antigenic peptide presented by MHC II to the T cell receptor (TCR) on CD4+ T cells initiates a cascade of signaling events that lead to T cell activation. This activation, in turn, stimulates the helper T cells to release cytokines, which play crucial roles in coordinating immune responses, activating other immune cells, and promoting antibody production by B cells.

On the other hand, MHC I molecules interact with CD8+ cytotoxic T cells. The binding of the antigenic peptide presented by MHC I to the TCR on CD8+ T cells triggers the activation of these cytotoxic T cells. Once activated, CD8+ T cells can directly kill infected or abnormal cells through the release of cytotoxic molecules, such as perforin and granzymes. This cytotoxic response is essential for eliminating intracellular pathogens and controlling tumor growth.

Conclusion

In summary, MHC II and MHC I molecules are critical players in the immune system, facilitating the presentation of antigens to T cells and initiating immune responses. While MHC II molecules are primarily expressed on APCs and present antigens derived from extracellular pathogens to CD4+ helper T cells, MHC I molecules are expressed on almost all nucleated cells and present antigens derived from intracellular pathogens or abnormal cellular components to CD8+ cytotoxic T cells. These distinct expression patterns, structures, and functions of MHC II and MHC I molecules allow for the coordination of different immune responses and contribute to the overall effectiveness of the immune system in protecting the body against infections and diseases.