Table 101' Cells participating in immune reactions

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• Table 10-1. Cells participating in immune
  reactions
• Lymphocytes  B cells Respond to cell-free and
  plasma membrane-bound antigens  
• T cells Respond to cell-bound antigens    
• Helper T cells Cytolytic T cells (CTLs)   
• Natural killer cells- Cell population lacking T
  cell receptor (TCR) and CD4 and CD8 co-receptors
• Accessory cells  Macrophages, Monocyte-derived
  cells  
• Dendritic cells Monocyte-derived cells (e.g.,
  Langerhans cells of the epidermis)  
• Follicular dendritic cells Found in lymphatic
  nodules
• Effector cells  Macrophages, CTLs, neutrophils 

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Table 10-2. Immunity Innate immunity
(natural)   Epithelia (physical
barrier)   Phagocytic cells (macrophages,
neutrophils)   Natural killer cells   Blood
proteins complement system Adaptive immunity
(also called acquired or specific
immunity)   Humoral immunity (antibody-mediated)  
   B cells and plasma cells   Cell-mediated
immunity (also called cellular immunity)     T
cells Types of immunity Passive
immunity   Maternal antibodies transferred to
fetus   Antibodies of immunized animals (rabies,
tetanus)   Antitoxins (diphtheria) Active
immunity (post disease)   T cells
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• Both humoral and cell-mediated immunity developed
  against foreign pathogens have the following
  characteristics
• Specificity Specific domains of an antigen are
  recognized by individual lymphocytes. We will see
  later how cell membrane receptors on lymphocytes
  can distinguish and respond to subtle variations
  in the structure of antigens.
• Diversity Lymphocytes utilize molecular
  mechanisms to modify their antigen receptors in
  such a way that they can recognize and respond to
  a large number and types of antigenic domains.
• Memory The exposure of lymphocytes to an antigen
  results in two events their antigen-specific
  clonal expansion by mitosis, as well as the
  generation of reserve memory cells. Memory cells
  can react more rapidly and efficiently when
  exposed again to the same antigen.
• Self-limitation An immune response is stimulated
  by a specific antigen. When the antigen is
  neutralized or disappears, the response ceases.
• Tolerance An immune response pursues the removal
  of a non-self antigen while being "tolerant" to
  self-antigens. Tolerance is achieved by a
  selection mechanism that eliminates lymphocytes
  expressing receptors specific for self-antigens.
  A failure of self-tolerance (and specificity)
  leads to a group of disorders called autoimmune
  diseases.

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• Box 10-A CD antigens
• Cell surface molecules recognized by mono-clonal
  antibodies are called antigens. These antigens
  are markers that enable the identification and
  characterization of cell populations. A surface
  marker that identifies a member of a group of
  cells, has a defined structure, and is also
  recognized in other members of the group by a
  monoclonal antibody is called a cluster of
  differentiation (CD).
• A helper T cell, which expresses the CD4 marker,
  can be differentiated from a killer T cell, which
  does not contain CD4 but expresses the CD8
  marker.
• CD markers permit the classification of T cells
  that participate in inflammatory and immune
  reactions.
• CD antigens promote cell-cell interaction and
  adhesion as well as signaling leading to T cell
  activation.

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• Box 10-B HIV reproductive cycle
• The life cycle of a retrovirus begins when the
  virus binds and enters a cell and introduces its
  genetic material (RNA) and proteins into the
  cytoplasm.
• 2. The genome of a typical retrovirus includes
  three coding regions gag, pol, and env,
  specifying, respectively, proteins of the viral
  core, the enzyme reverse transcriptase, and
  constituents of the viral coat.
• 3. In the cytoplasm, reverse transcriptase
  converts viral RNA into DNA which is then
  inserted into cellular DNA. This process is
  called integration.
• 4. The provirus DNA directs the synthesis of
  viral proteins and RNA.
• 5. The proteins enclose the RNA, forming viral
  particles that bud from the cell.

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• The complement system has the following specific
  characteristics important to remember
• Complement fragments C3a and C5a, produced by the
  enzymatic cascade, have proinflammatory activity.
  
• Complement fragment C3a and C5a recruit
  leukocytes to the infection site, which become
  activated and activate other cells.
• Other fragments (C3b and C4b), mark targets for
  destruction by phagocytes.
• Destruction of a pathogen is mediated by the
  final assembly of the MAC, a transmembrane
  cytolytic pore.
• Complement regulators (CRegs for example, CD55,
  CD46, and CD59) regulate the production of
  complement fragments, accelerate the decay of the
  already produced fragments, and block the final
  cytolytic action of the MAC by preventing its
  assembly. CRegs are cell surface-anchored
  proteins that protect host cells from unintended
  damage by the activated complement cascade. CD59
  blocks the destructive action of the MAC by
  preventing the binding of C9 to C8. CD59 also
  modulates the activity of T cells.
• Paroxysmal nocturnal hemoglobinuria (PNH)
  determines episodes of hemolysis represented by
  dark urine and anemia, stomach and back pain, and
  formation of blood clots. Red blood cells lack
  CD59 and are susceptible to destruction by the
  complement system. Therapeutic means to prevent
  or arrest the complement cascade are being
  developed to treat patients with PNH.

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• Box 10-C Dendritic cells and lymph nodes
• Terminal afferent lymphatic vessels, transporting
  lymph to the lymph nodes, derive from collecting
  lymphatic vessels.
• Terminal afferent lymphatic vessels penetrate the
  connective tissue cortex of a lymph node and
  empty their content in the subcapsular sinus.
• The flow of lymph into lymph nodes is regulated
  by smooth muscle cells present in the wall of
  collecting lymphatic vessels (intrinsic pumping
  activity) and by movements in the surrounding
  tissue (passive extrinsic activity).
• Collecting lymphatic vessels have valves that
  allow unidirectional flow of lymph and cells (for
  example, dendritic cells and leukocytes) from
  lymph node to lymph node. Valves prevent the
  backflow of lymph processed in the preceding
  lymph node.
• Dendritic cells are highly mobile. They are
  distributed as sentinels in the periphery to
  monitor the presence of foreign antigens. They
  relocate to secondary lymphoid organs, lymph
  nodes in particular, to interact with memory T
  cells present in the deep cortex. An example is
  the Langerhans cell present in epidermis.

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• The medulla contains two major components
• Medullary sinusoids, spaces lined by endothelial
  cells surrounded by reticular cells and
  macrophages.
• Medullary cords, with B cells, macrophages, and
  plasma cells. Activated B cells migrate from the
  cortex as plasma cells and enter the medullary
  sinuses. This is a strategic location because
  plasma cells can secrete immunoglobulins directly
  into the medullary sinuses without leaving the
  lymph node.

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