classical pathway

Examples of classical pathway in the following topics:

• Role of the Complement System in Immunity

  • The classical complement pathway is the main pathway by which the complement system occurs.
  • The alternative pathway is thought to be a leftover evolutionary precursor to the classical pathway.
  • Unlike the classical pathway, the alternative pathway is generally activated by microbial inflammatory mediators instead of antibodies.
  • Similarly to the classical pathway, C3b forms a C42b complex, and the rest of the steps are essentially the same as the classical pathway, ending with C5b forming a membrane attack complex with C6, C7, C8, and C9.
  • C4b creates C3 convertase, and the rest of the steps happen identically to the classical pathway from the C3 convertase step.

• The Complement System

  • Three biochemical pathways activate the complement system: the classical complement pathway, the alternative complement pathway, and the lectin pathway.
  • n the classical pathway, C1 binds with its C1q subunits to Fc fragments (made of CH2 region) of IgG or IgM, which forms a complex with antigens.
  • shows the classical and the alternative pathways with the late steps of complement activation schematically.
  • In the classical pathway, C4 binds to Ig-associated C1q and C1r2s2 enzyme cleaves C4 to C4b and 4a.
  • The classical and the alternative pathways with the late steps of complement activation.

• The Complement System

  • Activation of the complement leads to robust and efficient proteolytic cascades, which terminate in opsonization and lysis of the pathogen as well as in the generation of the classical inflammatory response through the production of potent proinflammatory molecules.
  • The complement system can be activated through three major pathways: classical, lectin, and alternative.
  • Initiation of the classical pathway occurs when C1q, in complex with C1r and C1s serine proteases (the C1 complex), binds to the Fc region of complement-fixing antibodies (generally IgG1and IgM) attached to pathogenic surfaces.
  • Further study in animals bearing natural complement deficiencies implicated the classical pathway as a crucial mechanism for efficient antigen trapping and retention in lymphoid tissues (e.g., splenic follicles), suggesting that a major function of the complement system was to localize foreign antigens into immune sites important for lymphocytes responses.

• Type II (Cytotoxic) Reactions

  • IgG and IgM antibodies bind to these antigens to form complexes that activate the classical pathway of complement activation to eliminate cells presenting foreign antigens (which are usually, but not in this case, pathogens).
  • The membrane attack complex (MAC; ) is typically formed on the surface of pathogenic bacterial cells as a result of the activation of the alternative pathway and the classical pathway of the complement system, and it is one of the effector proteins of the immune system.

• Steroids

  • Steroid biosynthesis is an anabolic metabolic pathway that produces steroids from simple precursors.
  • A unique biosynthetic pathway is followed in animals compared to many other organisms, making the pathway a common target for antibiotics and other anti-infective drugs.
  • The non-mevalonate pathway or 2-C-methyl-D-erythritol 4-phosphate/1-deoxy-D-xylulose 5-phosphate pathway (MEP/DOXP pathway) of isoprenoid biosynthesis is an alternative metabolic pathway leading to the formation of isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP).
  • The classical mevalonate pathway or HMG-CoA reductase pathway is an important cellular metabolic pathway present in all higher eukaryotes and many bacteria.
  • In contrast to the classical mevalonate pathway of isoprenoid biosynthesis, plants and apicomplexan protozoa such as malaria parasites have the ability to produce their isoprenoids (terpenoids) using an alternative pathway, the non-mevalonate pathway, which takes place in their plastids.

• The Entner–Doudoroff Pathway

  • The Entner–Doudoroff pathway is an alternate series of reactions that catabolize glucose to pyruvate.
  • The Entner–Doudoroff pathway describes an alternate series of reactions that catabolize glucose to pyruvate using a set of enzymes different from those used in either glycolysis or the pentose phosphate pathway .
  • Most bacteria use glycolysis and the pentose phosphate pathway.
  • This pathway was first reported in 1952 by Michael Doudoroff and Nathan Entner.
  • There are a few bacteria that substitute classic glycolysis with the Entner-Doudoroff pathway.

• Basic Principles of Classical Conditioning

  • Classical conditioning occurs when an unconditioned response becomes a conditional response to an unrelated conditional stimulus.
  • Classical conditioning is a form of learning whereby a conditioned stimulus (CS) becomes associated with an unrelated unconditioned stimulus (US), in order to produce a behavioral response known as a conditioned response (CR).
  • When a dog hears a buzzer and at the same time sees food, the auditory stimuli activates the associated neural pathways.
  • However, since these pathways are being activated at the same time as the other neural pathways, there are weak synapse reactions that occur between the auditory stimuli and the behavioral response.
  • Over time, these synapses are strengthened so that it only takes the sound of a buzzer to activate the pathway leading to salivation.

• Coagulation

  • The coagulation cascade is classically divided into three pathways: the contact (also known as the intrinsic) pathway, the tissue factor (also known as the extrinsic pathway), and the common pathway.
  • Both the contact pathway and the tissue factor feed into and activate the common pathway.
  • The intrinsic pathway (contact activation pathway) occurs during exposure to negatively charged molecules, such as molecules on bacteria and various types of lipids.
  • The extrinsic pathway occurs during tissue damage when damaged cells release tissue factor III.
  • In the final common pathway, prothrombin is converted to thrombin.

• Basic Principles of Classical Conditioning: Pavlov

  • Through his research, he established the theory of classical conditioning.
  • If we look at Pavlov's experiment, we can identify the four factors of classical conditioning at work:
  • With these results, Pavlov established his theory of classical conditioning.
  • When a dog hears a buzzer and at the same time sees food, the auditory stimulus activates the associated neural pathways.
  • However, because these pathways are being activated at the same time as the other neural pathways, there are weak synapse reactions that occur between the auditory stimulus and the behavioral response.

• Metabolic Pathways

  • An anabolic pathway requires energy and builds molecules while a catabolic pathway produces energy and breaks down molecules.
  • Consequently, metabolism is composed of these two opposite pathways:
  • One example of an anabolic pathway is the synthesis of sugar from CO2.
  • Chemical reactions in metabolic pathways rarely take place spontaneously.
  • Both types of pathways are required for maintaining the cell's energy balance.