phagocytosis, recognition

Definition

Target type: biologicalprocess

The initial step in phagocytosis involving adhesion to bacteria, immune complexes and other particulate matter, or an apoptotic cell and based on recognition of factors such as bacterial cell wall components, opsonins like complement and antibody or protein receptors and lipids like phosphatidyl serine, and leading to intracellular signaling in the phagocytosing cell. [GOC:curators, ISBN:0781735149]

Phagocytosis is a critical process in innate immunity, where specialized cells, primarily macrophages and neutrophils, engulf and destroy pathogens, cellular debris, and other foreign particles. Recognition of these targets is the first step in this complex process, and it involves a series of intricate interactions between the phagocyte and the target. The following describes the process of recognition in phagocytosis:

1. **Pattern Recognition Receptors (PRRs):** Phagocytes express a variety of PRRs on their surface, which act as sentinels for pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). These patterns are unique to pathogens or damaged cells, respectively. Some common PRRs include:
* **Toll-like receptors (TLRs):** TLRs recognize a wide array of PAMPs, including lipopolysaccharide (LPS) from bacteria, flagellin from bacteria, and viral dsRNA.
* **Scavenger receptors:** Scavenger receptors bind to a diverse range of molecules, including oxidized lipoproteins, bacterial components, and apoptotic cells.
* **C-type lectin receptors (CLRs):** CLRs bind to carbohydrates found on pathogens and damaged cells, such as mannose and glucans.
* **Complement receptors:** These receptors recognize complement proteins that have been deposited on the target's surface, indicating the presence of a pathogen or damaged cell.
2. **Opsonization:** Opsonization is a process that enhances phagocytosis by coating the target with molecules that bind to receptors on the phagocyte. The most common opsonins include:
* **Antibodies:** Antibodies are produced by B lymphocytes and bind specifically to antigens on the pathogen's surface. Phagocytes express Fc receptors (FcRs) that bind to the Fc portion of antibodies, facilitating phagocytosis.
* **Complement proteins:** Complement proteins are part of the complement system, a cascade of proteins that is activated when a pathogen is detected. Complement proteins can directly bind to the pathogen or can be deposited on the pathogen's surface by other complement proteins. Phagocytes express complement receptors (CRs) that bind to these complement proteins.
3. **Signal Transduction:** Once a PRR or opsonin binds to its target, a signal transduction cascade is initiated within the phagocyte. This cascade involves a series of intracellular signaling molecules, including kinases, phosphatases, and GTPases. The signal ultimately leads to the activation of downstream pathways that promote phagocytosis.
4. **Phagocytosis:** Upon successful recognition and signal transduction, the phagocyte extends pseudopodia, which are arm-like projections of the plasma membrane. These pseudopodia surround the target, engulfing it into a membrane-bound vesicle called a phagosome. The phagosome then fuses with lysosomes, which are organelles containing hydrolytic enzymes. These enzymes break down the engulfed pathogen or cellular debris, effectively eliminating the threat.

In summary, recognition in phagocytosis is a multi-faceted process that relies on the interplay of PRRs, opsonins, signal transduction pathways, and the dynamic cytoskeletal rearrangements that lead to phagocytosis. This intricate process ensures that phagocytes can efficiently identify and eliminate a wide range of threats, contributing to the host's defense against infection and disease.'
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