Complement J. Ochotná

Complement system of about 30 serum and membrane proteins (humoral component of nonspecific immunity) complement components in serum are present in inactive form complement activation has cascade character

complement proteins are synthesized in the liver, less by tissue macrophages and fibroblasts the main complement components: C1-C9 (C3 is the central component) other complement components: factor B, factor D, factor P regulatory proteins: C1 - inhibitor, factor I, factor H, C4bp, DAF, MCP, CR1, factor S (vitronectin), CD59 (protektin), inactivator of anafylatoxin

Functions of complement Opsonization (C3b, C4b) Chemotaxis (C3a, C5a) Osmotic lysis (MAC C5b-C9) Anafylatoxins (C3a, C4a, C5a)

Complement activation Alternative pathway Classical pathway Lektin pathway

Riedemann N.C.

Alternative pathway C3 component of complement rarely spontaneously breaks into C3b and C3a C3b can covalently bind on the surface of a particle (own cell, microorganism) or reacts with water and inactivate to bound C3b joins a factor B, which is cleaved by factor D to Ba and Bb, resulting complex C3bBb is stabilized by factor P and functions as an alternative C3 convertase

C3 convertase cleaves C3 to C3a (chemotactic for phagocytes) and C3b, which binds to the surface of the particles (opsonization), or gives rise to other C3 convertases from some C3 convertases form C3bBbC3b that act as an alternative C5 convertase, which cleaves C5 to C5a (chemotaxis) and C5b (starts terminal lytic phase)

Classical pathway can be initiated by antibodies (IgG, not by IgG4; IgM) or so-called pentraxins (CRP, SAP - acute phase proteins) after binding of antibodies to the bacteria surface, there is a change in its conformation and C1 protein can bind C1 have to bind to the 2 molecules of antibodies, change their conformation and get proteolytic activity - will cleave proteins C4 and C2

fragments C4b and C2a bind to the surface of organism and create the classic C3 convertase (C4bC2a), which cleaves C3 to C3a and C3b then creates a classic C5 convertase (C4bC2aC3b) that cleaves C5 to C5a and C5b

Zde dřívější nomenklatura – záměna 2a za 2b

Lectin pathway is initiated by serum mannose binding lectin (MBL) MBL binds to carbohydrate structures on the surface of some microbes, after the binding starts cleave C4 and C2 this way is similar to the classical pathway

Terminal (lytic) phase of the complement cascade C5b fragments creates a complex with C6, C7 and C8, the complex dive into the lipid membrane of the cell and attached to it into a circle 13-18 molecules of C9 (MAC – membrane attack complex), thus create in the membrane pores and cell can lysis (G-bacteria, protozoans, some viruses). Most microorganisms are resistant to this lytic effect of complement (protection by cell wall).

Regulation of complement and protection of own cells Activation of complement cascade is controlled by the plasma and membrane inhibitors. C1 inhibitor (C1-INH – inhibits C1, when the deficit → HAE ) DAF (decay-accelerating protein)-degradation of C3 and C5 convertase

factor I, MCP (membrane cofactor protein), CR1, factor H – C3b cleavage CD 59 (protectin) - prevents the polymerization of C9 factor S (vitronectin) – inhibits complex C5bC6 inactivator of anafylatoxins - inactivates anafylatoxins (C3a, C4a, C5a)

Complement receptors Bind fragments of complement components CR1 - on various cells - removing of immunecomplexes CR2 - on B lymphocytes and FDC - activation of B cells CR3, CR4 - on phagocytes - participation in opsonization, adhesion

4 basic complement functions Opsonization (C3b, C4b) Chemotaxis (C3a, C5a) Osmotic lysis (MAC C5b-C9) Anafylatoxins (C3a, C4a, C5a)

Antigens

Antigen (immunogen) substance that the immune system recognizes and responds to it usually proteins or polysaccharides (lipids and nucleic acids only in the combination with proteins or polysaccharides) molecules <5 kDa can´t trigger an immune response, the optimal size of the antigen molecules to initiate immune response is about 40 kDa

Hapten small molecules, that are able to induce specific immune response only after the establishment to the macromolecular carrier separate haptens are not immunogenic typically drugs (eg penicillin antibiotics, hydralazin)

Epitope = a part of the antigen which is recognized by immune system (lymphocytes- BCR, TCR, Ig)  linear epitope (AA sequence) - critical is amino acid composition  conformational epitope - critical is tertiary structure cross-reactive antigens - shares one or more identical or similar epitopes

Interaction antigen – antibody Binding site of antibody (paratop) form non-covalent complexes with the corresponding part on antigen molecule (epitope) participation: the hydrogen bonds, electrostatic and hydrophobic interactions, van der Waals forces antigen-antibody complex is reversible

Antigen endogenous antigens - autoantigens (self Ag) exogenous antigens - foreign substances from the environment allergen is exoantigen that in the susceptible individuals can cause pathological (allergic) immune response

Antigen features immunogenicity proteins> carbohydrates> macromolecule complexes (glycoproteins, nucleoproteins, and glycolipids)> lipids specificity

Factors affecting the immunogenicity Physical: solubility - insoluble more immunogenic molecular weight - ideal 5-40 kDa Chemical: structure - the number of determinants degradability - "ease" of uncovering the determinants in antigen presenting cells (APC cell) Biological: biological heterogeneity genetic and physiological disposition of the body

Degree of foreignness Autogeneic - antigens of the same individual Syngeneic - antigens of genetically identical individuals (eg twins) Allogeneic (alloantigens) - antigens genetically different individuals of the same species Xenogeneic (heterologous) - antigens derived from individuals of different species (eg monkey kidney transplant man)

Types of antigens according to antigen presentation thymus dependent antigens thymus independent antigens

Thymus dependent antigens more frequently, mostly protein Ag for specific humoral immune response to antigen is necessary to cooperate with TH lymphocytes (or response isn´t enough effective) assistance implemented in the form of cytokines produced by TH lymphocytes

Thymus independent antigens in a small number of antigens can be induced antibodies production directly without the participation of T lymphocytes this are mainly a bacterial polysaccharides, lipopolysaccharides and polymer forms of proteins (e.g. Haemophilus, Str.pneumoniae)

Superantigens proteins (microbial products) which have 2 binding sites, one interacts with the epitope presented on all MHCgpII, second interacts with other structures presented in many different TCR molecules (connection of T lymphocyte with APC) stimulate polyclonaly and massively massive activation of T lymphocytes can cause shock e.g. bacterial toxins (Staph.aureus, Str.pyogenes, Pseud.aeruginosa)

Differcence between antigen and superantigen binding

Sequestered antigens autoantigens that are normally hidden to immune system and therefore unknow (e.g. brain, the lens of the eye , testes) if they are "uncovered" by demage, exposed to the immune system, are recognized as foreign (one of the theories of autoimmune processes)

Immunologically privileged sites Some tissues in allogeneic transplant are far less rejected (eg CNS, cornea, gonads) Mechanisms of protection from the immune system: isolation from the immune system (blood-brain barrier) preferences Th2 and suppression of Th1-response active protection against effector T-lymphocytes This privileged position is not absolute

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