Complement Pathways: Types, Functions and Regulation

C4b can be inactivated by the action of C4BP, a plasmatic cofactor for FI (Figure 8B) (148–151). The complement system is a complex hub like network of proteins, reactions and processes. The recognition event induces a structural change in the recognition molecule, which in turn induces the activation of enzymes able to cleave the subsequent molecules in the cascade and generate the central enzymatic complexes of complement, CP and AP C3 convertases. It has special germ-fighting abilities.

C2b and C4b form C3 convertase and amplify the complement system (see alternate pathway). The C1s subunit then cleaves C4 and C2, and then two of the products, C4b and C2a, associate to form the C3-convertase, C4bC2a. Alternatively, C5 and C3 could be cleaved by proteases released from mast cells and other inflammatory cells (i. )The complement cascade can be activated via three different pathways: The binding site of the second C3 molecule is unclear, but it has been suggested to bind to the TED domain of C4d and to the CUB domain of C3b.

Humans have a single MBL (the product of MBL2; MBL1 is a pseudogene), CL-11 (collectin kidney 1, CL-K1) and three ficolins, FCN1 (M-ficolin), FCN2 (L-ficolin) and FCN3 (H-ficolin) [5], [8], [9].

Garg AD, Krysko DV, Verfaillie T, Kaczmarek A, Ferreira GB, Marysael T, et al. Despite the similarity between C1 and MBL/MASP complex architectures, the mechanism of activation of the CP and LP differ (37). Mechanism of C1 classical pathway activation and regulation. Unfortunately, the lack of coherence in complement proteins nomenclature and the complexity of the enzymatic cascade render complement one of the “most complicated and incomprehensible” parts of immunology and is frequently avoided by students and scientists. Although this approach is not specific for C3a, the results suggest that C3a does not influence either the nature or magnitude of the T cell response at the sensitization phase, but may regulate Th2 cytokine production and Th2-mediated effector functions through its ability to recruit and/or activate both Th2 effector cells and non–T cells capable of producing Th2 cytokines.

Contrary to Efb, staphylococcal complement inhibitor family (SCIN-A, SCIN-B, SCIN-C), can bind C3b in two distinct regions with a primary site at domain MG8 (191–194). Tumor-promoting inflammation has an important role in carcinogenesis and cancer progression (36–38). For these molecules, the available crystal structures were used for the visualization. Activation of C3a-C3aR signaling was shown to play an important role in guiding collective cell migration [77]. Membrane cofactor protein, DAF, and CR1 (CD35) serve as cofactors for FI-mediated proteolysis of C4b and C3b. Recognition of opsonic complement fragments, including Factor I-generated proteolytic fragments of C3b (iC3b, C3c, and C3dg), by phagocytic cells is accomplished through three families of CRs; the short consensus repeat module-containing CR1 and CR2, the β 2 integrin family members CR3 and CR4, and the immunoglobulin superfamily member CRIg 74, 75. CCP15–17 has a major role in C3b/C4b inactivation (146). C3 deposition on pneumococci was impaired, but not completely blocked, by C4 deficiency in both mice (fig. )

Complement fragments interact with each another to form functional complexes. Every effort was made to minimize suffering and mice were humanely culled if they became lethargic during infection experiments. It was named "complement" because the system helps antibody (defined) kill invaders. CR1 on erythrocytes plays a major role in the clearance of soluble immune complexes, by transporting them to the liver and spleen, where they are cleared by macrophages. How boost your immune system, according to an immunologist. By contrast, with traditional dominant–recessive gene combination like eye color, a single brown allele is dominant and if the other parent contributed a blue color allele, the eyes will be brown rather than a mix of brown and blue. Interestingly, the frequency of these SNPs is high, suggesting that these polymorphisms may have conferred evolutionary advantage in the past and perhaps in protection from parasitic infections. Racusen LC et al. At the same time, complement overactivation, due to the presence of anti-C1q antibodies or complement activating immune complexes in the circulation and in kidney glomeruli, is also leading to the same disease, albeit by a different mechanism.

Other Complement System and Immune System Products

Recent studies have shown that imbalanced complement activation and inflammation also triggered metastatic pathways in various cancer models by enhancing the motility of cancer cells, regulating the status of the TME, degrading the extracellular matrix (ECM) and disrupting tissue barriers. Where non-conjugated primary antibodies were used, the pneumococci were washed twice and incubated for a further hour with FITC conjugated anti-mouse IgG (Dako). Mice C3aR signaling on DCs is important for the generation of Th1 cells (28, 87). 15 tips to boost your child’s immunity, being exposed to germs is a part of life, and not necessarily unfavorable. Then, IL-6 regulates inflammation induced by bacteria. The fluid-phase C3 convertase cleaves C3 to C3a and C3b, creating sufficient C3b to bind with Bb and form C3bBb, which, when stabilized by Properdin on a microbial surface, acts as the principal C3 convertase of the alternative complement pathway. Since the atomic coordinates of the two C5 convertases have not been published yet, this figure represents the current model for their organization.

Clustering of IgG molecules on the antigen surface could be affected by antigen size, density, and fluidity (50, 51) such that smaller antigen-antibody complexes will allow only moderate complement activation.

Effectors Of The Complement System

The classical pathway, which has two components, globulin (C1) and albumin (C2) The alternate pathways Mannose-binding lectin. Pathogens have developed different strategies to inhibit complement activation. Vaccines that boost your immunity, she says, “you’ll be more likely to keep up with a daily habit if you actually enjoy the flavor. Further work by researchers such as Jules Bordet and Paul Erlich built on these early observations and developed various, at times competing, theories regarding the combined action of this heat-labile component, which we now know as complement, and heat-stable components (which later turned out to be antibodies) from the blood in killing pathogens.

It was called ‘complement’ because it aided the antibacterial activity of antibodies. Secondary antibodies were alkaline phosphatase-conjugates and bound antibody was detected using the colorimetric substrate p-nitrophenylphosphate (pNPP). Complex actin movements are then involved in the intracellular movement of the phagosome during its maturation to the phagolysosome. The exact contribution to T cells survival of the intracellular and extracellular C3a generation and C3aR signaling is still not well defined. Resting cells have only a few C3b molecules that are deposited and FH binds to them with the regulatory domains CCP1-4. The components of the complement system are heat labile, therefore specimens sent for complement assays must be handled carefully. Although asthma is multifactorial in origin, it is generally accepted that it arises as a result of inappropriate immunological responses to common environmental antigens in genetically susceptible individuals (1). FH has two main ligands, C3b and the GAG, found on host cells surface.

Understanding these mechanisms is important to facilitate the development of novel diagnostic and therapeutic strategies, and histopathological findings may contribute to conventional clinical and radiological factors. The alternative pathway can be activated either by the presence of foreign surfaces such as lipopolysaccharides or through C3b generated by spontaneous hydrolyses, the so-called “tick-over”. 6 ways to boost your immune system naturally, with mango, oranges, and greens, this Green Mango Orange Smoothie does just that. How chemotherapy affects the immune system, aside from containing anti-cancer properties, curcumin is also an anti-inflammatory and a pain reliever, he says. A lectin is a protein that binds sugars such as mannose. ’, Journal of immunology (Baltimore, Md. )It is a matter of debate whether the C1s catalytic domain faces the exterior or the interior of the C1q cone.

  • The explanation for the relative rarity of pneumonia and septicemia is poorly understood but is likely to be influenced by the host's innate immune response to S.
  • One molecule of C4b2a can cleave a large number of C3 molecules.


It binds directly to the costimulatory protein CD40 on human B cells at a site that differs from that used by the CD40 ligand. The diseases that accompany uncontrolled activation or inadequate performance of complement’s functions are often the result of inherited deficiency or subtle impairment of one or more of the components. The human body is composed to a myriad of different cells, forming a highly organized system. Binding of FH to CRP can enhance complement inhibition, particularly on apoptotic or damaged cells during inflammatory conditions (201). 1016/S0360-3016(99)00099-1. Prednisolone, weight-bearing activities such as walking, dancing, and running will help your muscles stay strong and healthy. Amarilyo G, Verbovetski I, Atallah M, Grau A, Wiser G, Gil O, et al.

The fragments generated by plasmin-mediated cleavage (C3a-like, C3b-like, iC3b-like, C3c-like, and C3dg-like) are similar, but not identical to fragments generated by the complement cascade and are biologically active. CR3 and to lesser extent CR4 are essential for phagocytosis of C3 fragments, opsonized immune complexes, and pathogens (304). Their proposed model could explain the strong antigen and epitope dependency of complement activation. Campbell WD, Lazoura E, Okada N, Okada H. Presentation of self-antigens by DC in the presence of C1q promotes the development of regulatory T cells (Treg) and the production of anti-inflammatory cytokines such as TGF-b, IL-10, PGE2, IL-37, and IL-27 and thus confers tolerance. This C3 convertase has enzymatic activity and is able to cleave the central complement component C3 to bioactive fragments C3a and C3b. Multiple complement regulators such as DAF, CR1, and FH decay the C3bBb complex formed on host cells. What happens then?

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IgG interacts with its target antigen forming specific circular structures. Indeed, complement is involved in TLR-induced inflammation (121) (Figure 5). Therefore, CP can be activated in either an immune complex-dependent and -independent manner. Besides the receptors for these anaphylatoxins, monocytes can also respond to C3b and iC3b via CR1, CR3 and CR4 70. These tiny filters are special blood vessels that clean the blood by letting wastes pass across their walls. Some sources designate the larger and smaller fragments as C2a and C2b respectively[14][15][16][17][18][19][20][21][22] while other sources apply the converse.

This chain reaction allows the system to respond very quickly against germs. C4b and CVF-C5 structures suggest that C4b-binding area for C5 is within the domains MG4, MG5, and MG7, supporting Laursen hypothesis. In addition, Lee and colleagues found that the mobilization of hematopoietic stem cells in C5-deficient mice was impaired and that C5a-mediated pro-mobilization effects were mediated by the stimulation of granulocytes rather than hematopoietic stem cells [96]. 15 tips to boost your child’s immunity, try to get a little sun daily to help boost your natural Vitamin D levels. All of this is a very important way for the body to fight off germs that may make us sick. These include neuromyelitis optica, atypical hemolytic uremic syndrome, and paroxysmal nocturnal hemoglobinuria. The proteins serve as a marker to indicate the presence of a pathogen to phagocytic cells, such as macrophages and B cells, and enhance engulfment; this process is called opsonization.

C1q is a six headed molecule, attached to two molecules C1r and C1s (seen enlarged in fig 1).

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While several complement proteins are produced by a wide variety of cell types, other complement proteins are produced by only a few related cell types. Formed convertase on CFHR4A had a higher resistance to FH-mediated decay. Nowadays, this vision has changed and it is well accepted that complement is a complex innate immune surveillance system, playing a key role in host homeostasis, inflammation, and in the defense against pathogens. C5a increases the cytokines production in immature DCs, but upon TLR-4 stimulation, C5a inhibits the production of IL-12, IL-23, and TNFα, thus having an anti-inflammatory role (127). These CFHRs, particularly CFHR1, are downregulators of FH, competing with FH for C3b and binding on the cell surface.

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The aHUS is a rare thrombotic microangiopathy that predominates in the kidney. C3a, C5a, and C5b–9 promote angiogenesis, helping in tumor nutrient support and dissemination. Factor B is an acute phase protein and increases during inflammation. The three pathways of the complement cascade The classical pathway is primarily activated by antigen-antibody complexes.

  • Complement activation generates C3a and C5a and MAC.
  • The C3(H 2 O)-bound FB is then cleaved by a serine protease (SP) Factor D (FD) allowing formation of a fluid phase C3 convertase complex C3(H 2 O)Bb.
  • In the mouse, C4 deficiency led to a significantly slower C3 turnover and in both species the absolute amount of C3 deposited on the bacteria was approximately half of that observed using WT serum.
  • VWF-A domain of CR3 binds to the TED domain (C3d part) of iC3b and the CUB and C345C domains of iC3b may also contribute to the interaction with the β-propeller and βI domains of the α-chain of CR3 (305).


’, The New England Journal of Medicine, 315(8), pp. Top 5 vitamins and minerals for a healthy immune system. C5b-9 deposits stably on rough and type 7 S. C5a, produced by this process, attracts macrophages and neutrophils and also activates mast cells. CFHR1, CFHR2, and CFHR5 share high homology in their two N-terminal domains, allowing them to form homo- and heterodimers (Figure 10) (203, 204). This occurs through three pathways; Classical pathway, activated by antigen-antibody reaction, Alternative pathway, activated on microbial cell surfaces, and Mannose binding Lectin pathway, activated by a plasma lectin that binds to mannose residues on microbes. Recent years have brought growing mechanistic awareness of the profound influence of the innate immune system on the development of adaptive immune responses. Castellano G, Woltman AM, Schlagwein N, Xu W, Schena FP, Daha MR, et al. It serves as a bridging molecule, facilitating the phagocytosis by dendritic cells or macrophages.

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The difference in the susceptibility of sheep and rabbit erythrocytes to lysis by human complement, which are at the basis of the classical hemolytic tests for complement activation, is expression level of sialic acid on the surface of these cells. Moreover, plasmin is able to bind the surface of S. For example, C1q exerts effects during pregnancy (where it is involved in remodelling of the maternal decidua), embryonic development, coagulation processes and neurological synapse function, which has been reviewed by Nayak et al. CR1 dissociates the C3 and C5 convertases. Exercise and immunity, our immune system can broadly be divided into two parts:. Moreover, it was shown that moDCs expressed C1q, C1s, C1r, C2, C3, C4, C5, C8, C9, FB, FD, FI, FH and FP at the mRNA level, and protein expression was confirmed for all except FI by using flow cytometry, Western blot and/or enzyme‐linked immunosorbent assay (ELISA) 88.

The small complement fragments (C3a, C4a, and C5a) are known as anaphylotoxins and are responsible for the local inflammatory responses generated following complement acitvation. The C3 convertases are able to bind C5, but with a very low affinity and cleavage rate. Studies of the C3b/C3d binding site on CCP19–20 showed that it may be overlapping, but it is not identical with the GAG-binding site (173–177). Although these interactions have not been formally explored in the pathogenesis of asthma, studies that explore the interaction of the complement system with other innate immune pathways are clearly warranted.

Crosstalk between C3aR and C5aR Signaling Pathways with Toll-Like Receptor Signaling

Then, CD46 plays a negative feedback when proliferation leads to T cell/T cell contact limiting the expansion of Th1 cells and allows a contraction phase. Activation of complement unfolds along three different complement activation pathways depending on the nature of the pathogen: Inherited and acquired quantitative and functional deficiency of the regulators of the alternative complement pathway FH, FI, and CD46 as well as abnormalities inducing overactivation of the AP C3 convertase are associated with rare renal diseases like aHUS (206, 207) and C3G (208, 209). Mutations in FB were located in multiple domains of the protein and in more than half of the cases they were far from any known binding sites (321). The axis C5a/C5aR appears to have a crucial role in sepsis, based on the observation that the inhibition of C5a/C5aR interaction decreased the mortality in a rat model (107). Leukocytes were washed twice with Hank's balanced salt solution (HBSS) containing 1. Depletion of CD8+ T cells in mice eliminated the protective effect of complement deficiency against tumor growth.

Aur is responsible for the consumption of C3 in the fluid phase (178). Muller-Eberhard, H. Complement activation, regardless of the pathway, converges on the generation of three broad effector pathways that serve to enable the complement to fulfill its physiological imperatives in host defense: This confers anti-inflammatory properties and supports tolerogenic apoptotic cell clearance. Experimental and structural analysis revealed that for FH and C3 almost all studied genetic changes mapped to the ligand binding site and had clear functional consequences.

Local complement production not only adds to the total pool of complement proteins that circulates, but influences other local processes via paracrine or autocrine interactions.

Classical Pathway

Here, we discuss the recent advances describing the molecular and structural basis of activation and regulation of the complement pathways and their implication on physiology and pathology. C3aR and C5aR1, respectively 45, 68, 69. It marks any cell which do not have certain protective proteins on their outsides. Following, the same enzyme will cleave C2 to generate the classical pathway C3 convertase C4b2a. Complement protects against infection in three ways. AP is permanently activated at low level by spontaneous hydrolysis of C3 into C3(H 2 O).

In summary, this study showed that the immunomodulatory effect of activated classical complement pathway inside tumors enhances tumor growth. Neil DA Hubscher SG (2020). 0000000000000368. To restore access and understand how to better interact with our site to avoid this in the future, please have your system administrator contact [email protected] (0/ ), which allows others to remix, tweak and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms. Bacteria were identified as pneumococci by Gram staining, catalase testing, alpha-hemolysis on blood agar plates, and determination of optochin sensitivity. C3a is an anaphylatoxin – a pro-inflammatory molecule, which activates the surrounding cells when reaching a threshold concentration. Nevertheless, deficiency of C5L2 on macrophages, neutrophils, and fibroblasts decreased their proinflammatory capacity in vitro.

Histamine causes increased vascular permeability and recruits other immune cells to the site of infection. C1q is the strongest genetic susceptibility factor for SLE, since over 90% of the individuals homozygous for mutations in this protein develop lupus-like symptoms (199). The emerging paradigm is that C3a production at the airway surface serves as a common pathway for the induction of Th2-mediated inflammatory responses to a variety of environmental triggers of asthma (i. )

Direct Killing

Complement proteins, C3 degradation products, and complement activation products (i. How ms may affect the immune system, acquired, or secondary, disorders you get later in life. )The studies above describe different mechanisms by which complement activation in the tumor microenvironment can enhance tumor growth: In C4 and C2 deficiency the risk is even less, and C3 deficiency is not a very strong risk factor for SLE 112, 113. Chen N-J, Mirtsos C, Suh D, Lu Y-C, Lin W-J, McKerlie C, et al. 6% for 12 mm to 40% for 25 mm. For example, C5a binding to C5aR decreases apoptosis in neutrophils (59) and T cells (22), and increases cell proliferation in endothelial (60) and colon cancer cell lines (61). Local synthesis of C3 by antigen presenting cells (APC) is implicated in antigen-specific T cell response. The name convertase indicates that this enzymatic complex can cleave (convert) a native C3 molecule into bioactive fragments C3a (the small fragment) and C3b (the big fragment).

  • Infection experiments Ten to twelve week old female MASP-2 and Fcna deficient mice, and their WT littermates were used.
  • Besides intracellular C3aR, a role was assigned more recently to intracellular C5aR1 cells in CD4+ cells by Arbore et al.
  • In such cases the IDF recommends that referral to an experienced immunologist to assist in the diagnostic evaluation be considered.
  • DCs present in human tissue stain positive for C1q 25, 85.
  • While complement is not adaptable it does complement the adaptive immune system and it is also involved in B and T cell response regulation.

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Unlike classical pathway, alternative pathway, does not require Ag-Ab complex for the initiation of complement pathway. In C3G, the complement system becomes over-active (too “revved up”). However, two important reasons justify studying the role of complement activation in cancer progression and the effect of complement manipulation in cancer therapy. This pathway is regulated by properdin, factor H, and decay-accelerating factor (CD55).

The complement system is so named because it is complementary to the antibody response of the adaptive immune system. This new enzyme loses its capacity to cleave C3 and starts to cleave C5. In addition, they point out that although less concentrated, complement deposits were found in unruptured aneurysms. Indeed, the importance of C3aR and C5aR expression for Th1 induction has been demonstrated in mice (27, 88, 89).

Immunology Of The Rat

C8 is the first component to penetrate the lipid bilayer upon interaction with the forming MAC. The AP monitors for pathogen invasion by maintaining a low level of constitutive activation by a process known as tick-over (4). This installs a vicious cycle of neutrophil and complement activation.

The classical pathway, the lectin or mannose binding lectin (MBL) pathway and the alternative pathway. However, its role remains unclear. Apoptosis, programed cellular death, is a process of normal cellular homeostasis and in healthy individuals everyday billions of cells die by this mechanism. Deficiencies of individual complement components or inhibitors of the system can lead to a variety of diseases (Table 1), which gives some indication of their role in protection against disease. Staphylokinase can form a complex with plasminogen, resulting in a serine protease activity efficient against C3 and IgG. It was hypothesized that the NLRP3 inflammasome in T cells receives signals via intracellular engagement of C5aR1, which increases the expression of IL1B and induces the production of reactive oxygen species (ROS), thereby activating the inflammasome 34. Thus, alterations in the balance of generation of the various components of the complement pathway either due to environmental exposure changes or genetic alterations in genes of the complement cascade may underlie the recent rise in asthma prevalence in westernized countries. In a transgenic mouse model of ovarian cancer, complement inhibition by C3 or C5aR knockout inhibited tumor growth by altering endothelial cell function and vascular endothelial growth factor (VEGF) expression [83].

  • If MAC is bound to the membrane, host cells can internalize it or remove it by ectocytose.
  • Several control proteins are also secreted by cultured monocytes, such as FH, FI, C4BP and C1INH 59, 62, 67.
  • Whether cell death is apoptotic or necrotic seems to be a function of C5b-9 quantity, and fragmentation of DNA has been detected in as little as 30 min after treatment with a lytic dosage of complement, illustrating the destructive potential of the terminal pathway of complement activation 45, 47.
  • Another complement receptor, complement receptor of the immunoglobulin superfamily (CRIg), binds to C3b and iC3b and also to soluble C3c 13, 14.
  • When these immune complexes lodge in blood vessels they can cause damage.

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The examples of systemic lupus erythematosus (SLE), atypical hemolytic uremic syndrome (aHUS), C3 glomerulopathies (C3G), age-related macular degeneration (AMD), and cancer are treated here in more details, but these are just few among a large list of diseases, including also the paroxysmal nocturnal hemoglobinuria (PNH), graft rejection after transplantation, ischemia/reperfusion injury, Alzheimer and Parkinson diseases, etc. There are three different complement pathways: Three different pathways, the classical, alternative and lectin pathways, mediate complement activation. Kupffer cells from CRIg-deficient mice are unable to efficiently clear C3-opsonized particles, resulting in increased mortality in the host in response to infection 83. In human, C3a-desArg is able to bind C5L2 and regulates triglyceride synthesis rate (110). Bound C4b will be inactivated by FI in presence of cofactors such as CR1 and/or MCP.

Early research has focused upon guinea pig and murine macrophages, where a wide variety of complement components are found to be expressed and/or secreted. The active C4bC2a activates C3. Like C1s, activated MASP-2 cleaves C4 and C4b-bound C2, generating C4b2a, the classical and lectin pathway C3 convertase. Natural foods and herbs for the immune system, most of our common herbs and spices originated in Asia or the Mediterranean region, where many were known healing foods and immune boosting folk remedies generations before manufactured drugs or supplements were derived from them. Immunity (2020) 39: Finally, C5 may mediate inhalational tolerance via recruitment and/or activation of pDCs as they have been shown to induce Tregs (CD4+CD25+ T cells).

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They found that the absence of C3 or CR3 made degeneration worse. Is it bullshit: that one minute of anger suppressed your immune system for 4-5 hours, while one minute of laughter boosts it for 24 hours? The immune system is traditionally divided into two parts, the innate immune system and the adaptive immune system. The N-terminal four CCPs contain the complement regulatory activity (132) and the CCP1–4 interacts with the MG ring of C3b. These activation fragments will be generated at sites of activation of the coagulation cascade. The complement itself can bind non-self pathogens after detecting their pathogen-associated molecular patterns (PAMPs),[21] however, utilizing specificity of the antibody, complements can detect non-self enemies much more specifically. It consists of three protein subunits Complement component 1 q subcomponent (C1q), Complement component 1 r subcomponent (C1r) and Complement component 1 s subcomponent (C1s). C4bC2a also serves as the C3 convertase in the lectin (mannose-binding) pathway, a complement pathway triggered when MBL or Ficolin bind to terminal mannose groups on the surfaces of microbes. In normal health, the complement system fights germs when they enter the body, and it does this without hurting the body itself.

If there is a pathogen, C3b binds to a protein in the blood called factor B, forming a C3b–Bb complex.


1007/s12282-015-0654-3. A functional abnormality of C1q, due to a mutation in the C1r2C1s2 binding site, resulted in an inability of C1 complex formation and presence of free C1q in the patient serum. The wastes go into the urine. CSCs are a subset of cells that possess the capacity to self-renew, differentiate, and give rise to cancer recurrence [91, 92]. CR2 activation by interaction with C3d-opsonized antigen on follicular dendritic cells increases CMH expression and allows antigen presentation to TCR. 3 C5 molecules per minute (214). Complement is part of the innate immune system.