enteroendocrine cells

Examples of enteroendocrine cells in the following topics:

• Intestinal Phase

  • Stretching of the duodenum accentuates vagal reflexes that stimulate the stomach, and peptides and amino acids in the chyme stimulate G cells of the duodenum to secrete more gastrin, which further stimulates the stomach.
  • Chyme also stimulates duodenal enteroendocrine cells to release secretin and cholecystokinin.
  • The enteroendocrine cells also secrete glucose dependent insulinotropic peptide.

• Gastric Phase

  • Histamine is a paracrine secretion from the enteroendocrine cells in the gastric glands.
  • Gastrin is a hormone produced by enteroendocrine G cells in the pyloric glands.
  • All three of these stimulate parietal cells to secrete hydrochloric acid and intrinsic factor.
  • The chief cells secrete pepsinogen in response to gastrin and especially Ach, and ACh also stimulates mucus secretion.
  • Below pH of two, stomach acid inhibits the parietal cells and G cells: a negative feedback loop that winds down the gastric phase as the need for pepsin and HCl declines.

• Cephalic Phase

  • Cephalic phase causes ECL cells to secrete histamine and increase HCl acid in the stomach.
  • There will also be an influence on G cells to increase gastrin circulation.
  • Chemical stimuli (i.e., partially digested proteins, caffeine) directly activate G-cells (enteroendocrine cells) located in the pyloric region of the stomach to secrete gastrin; this in turn stimulates gastric glands to secrete gastric juice.

• Specific T-Cell Roles

  • T helper cells assist the maturation of B cells and memory B cells while activating cytotoxic T cells and macrophages.
  • Differentiation into helper T cell subtypes occurs during clonal selection following T cell activation of naive T cells.
  • Cytotoxic T cells (TC cells, or CTLs) destroy virally infected cells and tumor cells, and are also cause much of the damage in in transplant rejection and autoimmune diseases.
  • Memory T cells comprise two subtypes: central memory T cells (TCM cells) and effector memory T cells (TEM cells), which have different properties and release different cytokines.
  • Regulatory T cells (Treg cells), also known as suppressor T cells, are crucial for the maintenance of immunological tolerance.

• Natural Killer Cells

  • Natural killer cells (NK cells) are cytotoxic lymphocyte critical to the innate immune system.
  • The role of NK cells is similar to that of cytotoxic T cells in the adaptive immune response.
  • NK cells provide rapid responses to virally infected cells and respond to tumor formation by destroying abnormal and infected cells.
  • NK cells use two cytolytic granule mediated apoptosis to destroy abnormal and infected cells.
  • Typically, immune cells detect major histocompatibility complex (MHC) presented on cell surfaces, triggering cytokine release and lysis or apoptosis in cells that do not express MHC I, or express much less of it than normal cells.

• Lymphoid Cells

  • The three major types of lymphocyte are T cells, B cells, and natural killer cells.
  • Therefore it acts as an inhibitor of NK cell activity, and NK cells will activate and destroy cells when it does not detect MHC class I on that cell surface.
  • Activated NK cells release cytotoxic (cell-killing) granules that contain perforin and granzyme, which can lyse cell membranes and induce apoptosis to kill infected or abnormal cells.
  • T cells are involved in cell-mediated immunity whereas B cells are primarily responsible for humoral immunity.
  • There are two types of T-cells involved in adaptive, cell-mediated immunity.

• Lymphocytes

  • T cells are called T cells because they mature in the thymus.
  • Subtype 2 helper T cells present antigens to B cells.
  • Suppressor T cells (T-reg cells)- T cells that kept some of their ability to bind to self-cells.
  • Plasma Cell- long-lived B cells that are the main source of antibodies.
  • Memory B Cells- dormant B cells that have the same BCR as the cell the B cell they differentiated from, and are specific to the antigen presented to that BCR.

• Maturation of T Cells

  • They can be distinguished from other lymphocytes, such as B cells and natural killer cells (NK cells), by the presence of a T cell receptor (TCR) on the cell surface.
  • T cells can be either helper T cells or cytoxic T cells based on whether they express CD4 (helper) or CD8 (cytotoxic) glycoprotein.
  • All T cells originate from haematopoietic stem cells in the bone marrow, which are capable of differentiating into any type of white blood cell.
  • A T cell is then signaled by the thymus to become a CD4+ cell by reducing expression of its CD8 cell surface receptors.
  • The remaining cells exit the thymus as mature naive T cells.

• Maturation of B Cells

  • B cells are lymphocytes that play a large role in the humoral immune response (as opposed to the cell-mediated immune response, which is governed by T cells) .
  • Once a B cell encounters its cognate antigen and receives an additional signal from a T helper cell, it can further differentiate into either plasma B cells or memory B cells.
  • B cells exist as clones.
  • A single B cell or a clone of cells with shared specificity, upon encountering its specific antigen, divides to produce many B cells.
  • B cells that encounter antigen for the first time are known as naive B cells.

• Clonal Selection and B-Cell Differentiation

  • B cells are lymphocytes that play a large role in the humoral immune response (as opposed to the cell-mediated immune response, which is governed by T cells).
  • B cells that have not been exposed to antigen, also known as naïve B cells, can be activated in a T cell-dependent or -independent manner.
  • T Cell Dependent Activation- activation of B cells by type 2 helper T cells in the lymph nodes.
  • B Cell differentiation refers to the process by which B cells change into different types, such as plasma cells and plasma blasts.
  • T cell-dependent B cell activation, showing a TH2-cell (left), B cell (right), and several interaction molecules