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Biological Functions of Lipopolysaccharide Antibodies
Published in Helmut Brade, Steven M. Opal, Stefanie N. Vogel, David C. Morrison, Endotoxin in Health and Disease, 2020
Antibodies and complement exhibit overlapping yet distinct opsonic functions (49,51). Recognition of surface-bound IgG by Fc receptors on phagocytic cells results in weak bacterial adherence, ingestion, and metabolic events associated with intracellular killing. Ligation of complement receptors by C3b and iC3b, on the other hand, produces more efficient adherence to phagocytes than that mediated by Fc receptors, but does not result in phagocytosis unless the complement receptors are first activated by an independent signal (52–55). Ligated complement receptors, even then, do not activate oxidative metabolism or microbicidal functions. LPS antibodies and serum complement thus serve complementary attachment and phagocytosis-promoting opsonic functions (51), resulting in collaborative mechanisms of opsonophagocytic killing.
Host Defense I: Non-specific Immunity
Published in Constantin A. Bona, Francisco A. Bonilla, Textbook of Immunology, 2019
Constantin A. Bona, Francisco A. Bonilla
The β2 integrins are also heterodimers whose a chains are different forms of CD11, and β chain is CD18. LFA-1 (CD11a/CD18) interacts with ICAM-1 (CD54) and ICAM-2 (CD102). This interaction is important for granulocyte function, as well as intercellular interactions important in T and B cell activation. CD11 b/CD18 and CD11 c/CD18 are also known as complement receptors CR3 and CR4, respectively. These bind the iC3b component of complement and assist in opsonization (see above). CD11b/CD18 also binds ICAM-1, and coagulation factor X. CD18 by itself binds to bacterial lipopolysaccharide.
The complement system in health and disease
Published in Gabriel Virella, Medical Immunology, 2019
Degradation of C3b by factor I and cofactors, such as CR1, results in the formation of two fragments, iC3b and C3dg. C3dg can be further degraded into C3d via nonspecific proteases, and all of these fragments are ligands for CR2. CR2 has primary binding specificity for a molecular site on the α-chain of C3 that is exposed during proteolytic cleavage of C3 to iC3b, C3dg, and C3d. B lymphocytes have both CR2 and CR1 molecules on their surface. Follicular dendritic cells (important in antigen presentation) have CR2, CR3, and CR1 on their surface. Antibody production is greatly enhanced by complement-coated antigens, which stimulate B cells via their CR2 and CR1. In animal models, when C3d was chemically linked to an antigen, and added to specific B cells in vivo, a thousand-fold enhancement of antibody production occurred. CR2 not only stimulates the B cells directly but also associates with CD19, another B-cell membrane protein that is known to greatly stimulate antibody production. Interestingly, CR2 is also the primary receptor for the immunoregulatory molecule CD23, an interaction that promotes the production of IgE, as well as the receptors for Epstein–Barr virus, involved in the induction of B-cell malignancies.
The Complement System in Retinal Detachment with Choroidal Detachment
Published in Current Eye Research, 2022
Shasha Luo, Yanghao Chen, Lufei Yang, Xuechun Gong, Zhifeng Wu
Complement regulatory protein DAF is a glycosylated membrane protein.15 To the best of our knowledge, this may be the first quantitative measurement of sDAF in the vitreous fluid of eyes with either RRDCD or RRD. An elegant study has shown that sDAF in urine has C4bp (or factor H) activity, indicating that it can inhibit the liquid phase activation of the complement cascade, which is equivalent to the role of serum C4–binding protein.16 Complement factor I (C3bINH) is an esterase that is a C3b inhibitory factor that can cleave and inactivate C3b to become the ineffective iC3B. It can also cleave C4b into C4d and C4c, thereby inhibiting the activation of the complement system.17 In the RRDCD group, the levels of sDAF and CFI in the vitreous humor were significantly increased, further indicating that eyes with RRDCD may still have normal complement regulation mechanisms, but that some persistent inflammatory mechanisms cause the continuous activation of the complement pathway and the level of complement inhibitory factors increase accordingly. Interestingly, the CFD and C2 were higher in the RRD group than in the control group, and the downstream components were not activated. This phenomenon may occur due to the increased levels of CFI.
High glucose-mediated overexpression of ICAM-1 in human vaginal epithelial cells increases adhesion of Candida albicans
Published in Journal of Obstetrics and Gynaecology, 2018
Hiroshige Mikamo, Yuka Yamagishi, Hiroyuki Sugiyama, Hisato Sadakata, Shun Miyazaki, Takako Sano, Tsutomu Tomita
Candida growth is stimulated and the virulence is increased under high glucose environments. This facilitates its colonisation of the genital tract. The unique virulence factors in Candida include a complement (iC3b) receptor, which increases in high-glucose concentrations (Hostetter 1990; Hostetter et al. 1990). This complement receptor has been shown to be structurally and functionally homologous to human integrin CD11b/CD18 (Hostetter 1990). CD11b/CD18 is also known as iC3b receptor, complement receptor type 3 (CR3) or Mac-1. It is expressed in granulocytes, monocytes and macrophages (Elangbam et al. 1997). When complement binds to Candida, it is usually phagocytosed. CD11b/CD18 on phagocytes recognises iC3b on the complement C3 fragment. However, the Candida complement receptor binds to iC3b and masks it. Thereby, the CD11b/CD18 on phagocytes is prevented from binding to iC3b on the Candida. As a result, Candida evades phagocytosis (Gilmore et al. 1988).
Acute post-streptococcal glomerulonephritis: analysis of the pathogenesis
Published in International Reviews of Immunology, 2021
Jesús Mosquera, Adriana Pedreañez
Three major types of effects after complement activation have been documented: a) Induction of anaphylatoxins (C3a and C5a), which attract and activate leukocytes. b) Induction of opsonins (C3b, iC3b, and C3d), which facilitate removal of target cells or immune complexes. c) The terminal membrane attack complex (MAC, C5b-9) formation that directly lyses targeted pathogens or damaged cells [33]. In most instances of APSGN activation of the complement occurs, as reflected by decreased levels of the complement proteins C3, C5, and properdin. Several studies implicate the membrane attack complex (MAC) in the pathogenesis of glomerular injury during APSGN (Figure 1B) [34,35].