Phagocytosis

Which receptors are involved in the phagocytosis of non-opsonized targets?

Several groups of receptors that recognize ligands directly are involved in the phagocytosis of non-opsonized targets. These include:

• Internalization receptors — mannose receptor (MR — CD206), DEC-205, Dectin-1.
• Scavenger receptors (scavengers) — SR-AI/II, CD36, which recognize bacterial lipid components and apoptotic cells.
• Receptors for apoptotic cells — phosphatidylserine receptor (PSR) and integrin $\alpha_v\beta_3$, which recognizes vitronectin.

Which molecules act as the main opsonins?

The primary opsonizing factors are:

• IgG class antibodies;
• complement C3 component fragment — iC3b.

IgG and iC3b often opsonize a cell jointly: antibodies within immune complexes activate the complement system, depositing C3b on the target cell, which is subsequently cleaved into iC3b.

Sources also mention C3b, complement component C1q, acute-phase proteins, mannose-binding lectin, LPS-binding protein, antimicrobial peptides, and extracellular matrix proteins, including fibronectin, as opsonins or opsonizing factors.

Which oxygen-dependent factors provide intracellular killing?

Oxygen-dependent factors of intracellular killing include reactive oxygen species generated during the respiratory burst:

• superoxide anion ($O_2^-$);
• hydrogen peroxide ($H_2O_2$);
• hydroxyl radical ($OH^\bullet$);
• hydroxide ion ($OH^-$);
• hypochlorite ion ($OCl^-$);
• singlet oxygen ($^1O_2$);
• ozone ($O_3$).

The myeloperoxidase microbicidal system in phagolysosomes includes myeloperoxidase, hydrogen peroxide, and halogen ions. In the presence of chloride, hypochlorous acid / hypochlorite ($HOCl/OCl^-$) is formed, and the interaction of $HOCl$ with amino acids yields chloramines, which exhibit bactericidal activity.

Which oxygen-independent factors participate in bacterial destruction inside the phagocyte?

The destruction of bacteria inside the phagocyte without oxygen involvement is mediated by local acidification of the medium, enzymes, and antimicrobial peptides. Oxygen-independent killing involves:

• Acidification factors — lower the pH to 4.5–5.0 through V-ATPase activity.
• Cationic proteins — serprocidins (cathepsin G, elastase, azurocidin, proteinase-3), lysozyme, lactoferrin, BPI proteins, and vitamin $B_{12}$-binding factor.
• Acid hydrolases — 5'-nucleotidase, $\beta$-arylsulfatase, $\beta$-glucuronidase, acid glycerophosphatase, and glycosidases.
• Bactericidal peptides — $\alpha$- and $\beta$-defensins, cathelicidins.
• Competitive metabolism inhibitors.

Which infections (by which pathogens) are frequently associated with incomplete phagocytosis?

According to the sources, incomplete phagocytosis is described for the following pathogens and infections:

• Gonococci: Por proteins prevent the fusion of the lysosome with the phagosome, ensuring gonococcal survival inside phagocytes; upon microscopy, gonococci may be located within leukocytes.
• Mycobacterium tuberculosis: within the phagocyte, they secrete factors that prevent the fusion of the phagosome with the lysosome; mycobacteria remain viable and multiply within the macrophage.
• Staphylococci: incomplete phagocytosis, i.e., bacterial survival inside phagocytes, leads to persistent bacteremia and metastatic purulent foci.
• Typhoid fever pathogen / acute intestinal infections according to the provided scheme: incomplete phagocytosis results in the pathogen penetrating through the thoracic lymphatic duct into the blood, leading to the development of bacteremia.

Which cells of the immune system are classified as professional phagocytes?

According to the sources, immune system cells with pronounced phagocytic activity include:

• neutrophils — they arrive first at the site of inflammation and realize a high protective potential in the form of phagocytosis;
• monocytes — they migrate following neutrophils, participate in phagocytosis, and differentiate into macrophages;
• macrophages — they perform phagocytosis, including pathogens and apoptotic cells.

The sources also indicate that endothelial and epithelial cells can acquire phagocytic capacity upon activation and are considered facultative phagocytes.

Which specific chemotactic factors attract phagocytes to the site of inflammation?

Phagocytes are attracted to the site of inflammation by exogenous and endogenous chemotactic factors.

Specific chemoattractants include:

• bacterial N-formyl peptides, including N-formyl-methionyl-leucyl-phenylalanine;
• anaphylatoxins C3a and C5a;
• leukotrienes;
• chemokines;
• pro-inflammatory cytokines with chemotactic activity;
• cleavage products of blood coagulation and fibrinolysis factors — thrombin, fibrin;
• neuropeptides;
• immunoglobulin fragments;
• pentraxins — C-reactive protein, serum amyloid;
• platelet-activating factor (PAF).