first class lever

(noun)

Force and load to either side of the pivot.

Related Terms

Examples of first class lever in the following topics:

  • Lever Systems

    • Classes of levers include:
    • In a first class lever, the load and force sit on either side of the pivot like a seesaw.
    • First-class levers are relatively uncommon in the body, but one example is the triceps brachii muscle of the upper arm which acts to extend the forearm.
    • Second-class levers are also relatively uncommon in the body.
    • In a third-class lever the force is applied between the load and the pivot.

  • Antigen-Presenting Cells

    • The type of T cell activated, and therefore the type of response generated, depends in part on the context in which the antigen was first encountered by the APC.
    • MHC Class I molecules present antigen to CD8+ cytotoxic T cells.
    • With the exception of some cell types (such as erythrocytes), Class I MHC is expressed by almost all host cells.
    • Exogenous antigens are usually displayed on MHC Class II molecules, which interact with CD4+ helper T cells.
    • Expression of Class II is more restricted than Class I.

  • 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.
    • In order for an antigen presenting cell (APC) to present an antigen to a naive T cell, it must first be processed into a form in which the antigenic determinant can be recognized by the T cell receptor.
    • 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.
    • The Exogenous Pathway- occurs when MHC class II molecules present fragments derived from extracellular (exogenous) proteins that are located within the cell.
    • First, pathogens are phagocytized, and then endosomes within the cell break down antigens with proteases, which then combine with MHC II.

  • Embryonic Development

    • The first step in CNS development is primary neurulation that divides the ectoderm into three tissue regions, the neural tube, which is internally located, the epidermis, which is externally located, and the neural crest cells, which develop in the region between the neural tube and epidermis but then migrate to new locations.
    • The neural tube becomes patterned along the dorsal-ventral axis to establish defined compartments of neural progenitor cells, which will give rise to distinct classes of neurons.
    • Class I are inhibited by Shh while Class II are activated by Shh.
    • These two classes of proteins then cross-regulate each other to create more defined boundaries of expression.
    • V0-V3 represent four different classes of ventral interneurons, and MN indicates motor neurons.

  • Complete Antigens and Haptens

    • The first haptens used were aniline and its carboxyl derivatives (o-, m-, and p-aminobenzoic acid).
    • Usually, the first exposure causes only sensitization, in which there is a proliferation of helper and cytotoxic T-cells.
    • This also appears to be the mechanism by which the anesthetic gas halothane can cause a life-threatening hepatitis, as well as the mechanism by which penicillin-class drugs cause autoimmune hemolytic anemia.

  • Phagocytes

    • There are many different classes of phagocytes within the body, that all perform different specialized functions involving phagocytosis.
    • They express MHC class II molecules which make them the ideal antigen presenting cell.
    • Neutrophils are a type of PMN granulocyte normally found in the bloodstream, and are the most abundant type of phagocyte, and the first responder during inflammation.
    • Mast cells express MHC class II molecules and can participate in antigen presentation; however, the mast cell's role in antigen presentation is not very well understood.

  • Lymphocytes

    • Helper T cells (CD4s)- facilitate the organization of immune responses, and can bind to MHC class II.
    • They function similarly to natural killer cells by binding to MHC class I and releasing perforin, granzymes, and proteases to induce apoptosis in a pathogen.
    • During antigen presentation, antigen presenting cells first present antigens to T cells.

  • Galactosemia and Glycogen Storage Disease

    • Detection of the disorder through newborn screening (NBS) does not depend on protein or lactose ingestion, and, therefore, it should be identified on the first specimen unless the infant has been transfused.
    • GSD has two classes of cause: genetic and acquired.

  • Types of Neurotransmitters by Function

    • Neurotransmitters fall into several chemical classes based on the molecular structure.
    • Acetylcholine, which acts on the neuromuscular junction, was the first neurotransmitter identified.

  • Maturation of B Cells

    • The B cell may either become one of these cell types directly or it may undergo an intermediate differentiation step, the germinal center reaction, where the B cell will hypermutate the variable region of its immunoglobulin gene ("somatic hypermutation") and possibly undergo class switching.
    • Most of such B cells differentiate into plasma cells that secrete antibodies into blood that bind the same epitope that elicited proliferation in the first place.
    • B cells that encounter antigen for the first time are known as naive B cells.