Question

What is the purpose of a major histocompatibility complex?

Short answer

The purpose of the major histocompatibility complex is to present peptide fragments from pathogens on the cell surface for recognition by T-cells.

Step-by-step solution

Identify the Major Histocompatibility Complex (MHC)

Understand what the Major Histocompatibility Complex (MHC) is. MHC is a set of cell surface proteins essential for the acquired immune system to recognize foreign molecules in vertebrates, which in turn determines histocompatibility.

Recognize two classes of MHC

There are two classes of MHC molecules: MHC class I and MHC class II. MHC class I molecules are found on all nucleated cells and present peptide fragments to cytotoxic T cells. MHC class II molecules are mainly found on antigen-presenting cells like macrophages, dendritic cells, and B cells, and they present processed antigens to helper T cells.

Understand the function of MHC

The primary purpose of MHC molecules is to bind peptide fragments derived from pathogens and display them on the cell surface for recognition by appropriate T-cells. This process is critical for the immune system to detect and respond to pathogens.

Key concepts

MHC class I

MHC class I molecules play a critical role in the immune response by presenting peptide fragments from intracellular pathogens, such as viruses, to cytotoxic T cells (also known as CD8+ T cells). These molecules are found on the surface of all nucleated cells in the body. When a cell is infected with a virus, it processes the viral proteins into smaller peptides. These peptides are then loaded onto MHC class I molecules and transported to the cell surface.
Cytotoxic T cells patrol the body, scanning cells for these MHC class I-peptide complexes. When a cytotoxic T cell recognizes a viral peptide presented by an MHC class I molecule, it binds to the infected cell and releases enzymes that kill the infected cell, preventing the virus from replicating. This process is essential for the control and elimination of intracellular infections.

MHC class II

MHC class II molecules are primarily found on specialized immune cells known as antigen-presenting cells (APCs), including macrophages, dendritic cells, and B cells. These molecules present peptide fragments from extracellular pathogens, like bacteria, to helper T cells (CD4+ T cells).
When an APC engulfs a pathogen, it processes the pathogen's proteins into peptide fragments within lysosomes. These peptides are then loaded onto MHC class II molecules and transported to the cell surface. Helper T cells recognize these MHC class II-peptide complexes and, upon binding, become activated.
Activated Helper T cells play a vital role in orchestrating the immune response by releasing cytokines that stimulate other immune cells, including B cells, which produce antibodies, and cytotoxic T cells, which kill infected cells. Thus, MHC class II molecules are key players in initiating and regulating the body's defense against extracellular pathogens.

Immune Response

The immune response is the body's defense mechanism against foreign invaders like viruses, bacteria, and other pathogens. MHC molecules are essential components in this process, as they enable the immune system to distinguish between self and non-self.
There are two main types of immune responses: the cell-mediated immune response and the humoral immune response. The cell-mediated immune response involves cytotoxic T cells that directly attack and destroy infected cells, while the humoral immune response involves B cells that produce antibodies to neutralize pathogens.
MHC class I molecules are involved in the cell-mediated immune response by presenting peptides from intracellular pathogens to cytotoxic T cells. In contrast, MHC class II molecules are involved in the humoral immune response by presenting peptides from extracellular pathogens to helper T cells, which then activate B cells and other immune cells.
Both MHC class I and MHC class II molecules are crucial for the recognition of pathogens and the coordination of an effective immune response, ensuring that the body can detect and eliminate harmful invaders while maintaining tolerance towards its own tissues.