Allorecognition

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  Allorecognition Triggers Autophagy and Subsequent Necrosis in the Cnidarian Hydractinia

Allorecognition

Allorecognition is a biological process through which an organism is able to distinguish its own cells and tissues from those of another organism of the same species. This ability is crucial for the immune system to function properly, as it allows the organism to identify and respond to foreign cells, such as those from pathogens or transplanted tissues.

Mechanisms of Allorecognition

Allorecognition involves several complex mechanisms that vary across different species. In vertebrates, the primary system responsible for allorecognition is the Major Histocompatibility Complex (MHC). The 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.

Major Histocompatibility Complex (MHC)

The MHC is divided into two main classes: MHC class I and MHC class II. MHC class I molecules are present on almost all nucleated cells and present peptide fragments derived from intracellular proteins. MHC class II molecules are found on antigen-presenting cells such as dendritic cells, macrophages, and B cells, and present peptides derived from extracellular proteins.

T Cell Receptors (TCR)

T cell receptors are crucial for the recognition of antigens presented by MHC molecules. Each T cell has a unique TCR that can bind to a specific antigen-MHC complex. This binding is essential for the activation of T cells, which play a central role in the immune response.

Natural Killer (NK) Cells

Natural killer cells are a type of lymphocyte that can recognize and destroy cells that do not express "self" MHC molecules. This is particularly important in the context of viral infections and tumor surveillance.

Allorecognition in Transplantation

In the context of organ transplantation, allorecognition is a critical factor that can lead to graft rejection. The recipient's immune system recognizes the donor tissue as foreign due to differences in MHC molecules, leading to an immune response against the transplanted organ.

Types of Graft Rejection

There are three main types of graft rejection:

• Hyperacute rejection: Occurs within minutes to hours after transplantation, usually due to pre-existing antibodies against the donor's antigens.
• Acute rejection: Occurs days to weeks after transplantation and involves T cell-mediated immune responses against the donor tissue.
• Chronic rejection: Develops over months to years and is characterized by a gradual loss of graft function due to long-term immune-mediated damage.

Allorecognition in Invertebrates

Invertebrates also exhibit allorecognition, although the mechanisms differ from those in vertebrates. For example, colonial marine invertebrates such as sponges and corals use allorecognition to prevent fusion with genetically different colonies, which can lead to competitive interactions and resource allocation issues.

Related Pages

• Immune system
• Transplant rejection
• Histocompatibility
• Antigen presentation