The Exogenous Pathway

(noun)

Phagocytized pathogens are broken down from within the cell and their broken-down antigens bind with MHC II, which then is expressed on the surface of the antigen-presenting cell.

Related Terms

Examples of The Exogenous Pathway in the following topics:

  • Antigenic Determinants and Processing Pathways

    • The determinants need not be located on the exposed surface of the antigen in its original form, since recognition of the determinant by T cells requires that the antigen be first processed by antigen presenting cells.
    • However, the pathway leading to the association of protein fragments with MHC molecules differs between class I and class II MHC, which are presented to cytotoxic or helper T cells respectively.
    • There are two different pathways for antigen processing:
    • The endogenous pathway occurs when MHC class I molecules present antigens derived from intracellular (endogenous) proteins in the cytoplasm, such as the proteins produced within virus-infected cells.
    • The exogenous pathway occurs when MHC class II molecules present fragments derived from extracellular (exogenous) proteins that are located within the cell.

  • Fever

    • PGE2 release comes from the arachidonic acid pathway, which also produces inflammatory mediators such as thromboxane and leukotriene.
    • This pathway is mediated by the enzymes phospholipase A2 (PLA2), cyclooxygenase-2 (COX-2), and prostaglandin E2 synthase.
    • A pyrogen is a substance that induces fever and can be either internal (endogenous) or external (exogenous) to the body.
    • Exogenous factors s lipopolysaccharide toxin (from gram negative bacteria) which can activate a number of innate immune activation pathways.
    • When TNFα or any of these cytokine factors bind to cells in phospolipids in the brain, the arachidonic acid pathway is activated and PGE2 released to act on the hypothalamus and cause the fever response.

  • Antigen-Presenting Cells

    • The ability of the adaptive immune system to survey for infection requires specialized pathways of enabling recognition of pathogen-derived antigens by T cells.
    • Dendritic cells (DCs) phagocytose exogenous pathogens such as bacteria, parasites, and toxins in the tissues and then migrate, via chemotactic signals, to T cell-enriched lymph nodes.
    • Exogenous antigens are usually displayed on MHC Class II molecules, which interact with CD4+ helper T cells.
    • In the upper pathway; foreign protein or antigen (1) is taken up by an antigen-presenting cell (2).
    • In the lower pathway; whole foreign proteins are bound by membrane antibodies (5) and presented to B lymphocytes (6), which process (7) and present antigen on MHC II (8) to a previously activated T helper cell (10), spurring the production of antigen-specific antibodies (9).

  • Antigens and Antigen Receptors

    • Non-microbial exogenous (non-self) antigens can include pollen, egg white, and proteins from transplanted tissues and organs or on the surface of transfused blood cells.
    • Exogenous antigens are antigens that have entered the body from the outside, for example by inhalation, ingestion, or injection.
    • The immune system's response to exogenous antigens is often subclinical.
    • By endocytosis or phagocytosis, exogenous antigens are taken into the antigen-presenting cells (APCs) and processed into fragments.
    • Some antigens start out as exogenous antigens, and later become endogenous.

  • Histology of the Liver and Gallbladder

    • Hepatocytes are the main tissue cells of the liver; the gallbladder contains the mucosa, muscularis, perimuscular, and serosa layers.
    • A hepatocyte is the main tissue cell of the liver, making up 70-80% of the liver's cytoplasmic mass.
    • Hepatocytes are involved in protein synthesis, protein storage, transformation of carbohydrates, synthesis of cholesterol, bile salts and phospholipids, and detoxification, modification, and excretion of exogenous and endogenous substances.
    • The liver is capable of complete regeneration from as little as 25% of the original organ.
    • Describe the layers of the gallbladder and the histology of the liver

  • 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.
    • In the final common pathway, prothrombin is converted to thrombin.
    • Tissue factor pathway inhibitor (TFPI): limits the action of tissue factor (TF) and the factors it produces.

  • Role of the Complement System in Immunity

    • The classical complement pathway is the main pathway by which the complement system occurs.
    • The alternative pathway may 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.
    • The steps for the alternative pathway are:
    • The steps for the lectin pathway are:

  • Organization of Motor Neuron Pathways

    • The motor system is the part of the central nervous system that is involved with movement.
    • The motor pathway, also called the pyramidal tract or the corticospinal tract, serves as the motor pathway for upper motor neuronal signals coming from the cerebral cortex and from primitive brainstem motor nuclei.
    • The axons of these cells pass from the cerebral cortex to the midbrain and the medulla oblongata.
    • This diagram of spinal cord tracts shows the motor and efferent pathways in red and the sensory and afferent pathways in blue.
    • Included in the diagram are the following motor pathways: corticospinal tracts (pyramidal tract), and extrapyramidal tracts (tectospinal tract not delineated).

  • Somatic Sensory Pathways

    • The somatosensory pathway is composed of three neurons located in the dorsal root ganglion, the spinal cord, and the thalamus.
    • A somatosensory pathway will typically have three long neurons: primary, secondary, and tertiary.
    • In the spinal cord, the somatosensory system includes ascending pathways from the body to the brain .
    • This is the target for neurons of the dorsal column–medial lemniscal pathway and the ventral spinothalamic pathway.
    • Note that many ascending somatosensory pathways include synapses in either the thalamus or the reticular formation before they reach the cortex.

  • Sensory and Motor Tracts

    • The spinothalamic tract is a sensory pathway originating in the spinal cord.
    • The pathway decussates at the level of the spinal cord.
    • Somatosensory organization is divided into the dorsal column–medial lemniscus tract (the touch/proprioception/vibration sensory pathway) and the anterolateral system, or ALS (the pain/temperature sensory pathway).
    • Both sensory pathways use three different neurons to get information from sensory receptors at the periphery to the cerebral cortex.
    • In both pathways, primary sensory neuron cell bodies are found in the dorsal root ganglia, and their central axons project into the spinal cord.