Immuno Lec 6 - Immunological Organs and Cells

Question 1

lymphocytes; subsets and part of immune system

Answer 1

-T cells and B cells = adaptive immune system

-natural killer cells = historically known with the innate immune system

Question 2

lymphocytes; develop from what, primary and secondary tissues

Answer 2

• Develop from stem cells in the bone marrow called common lymphoid progenitors
• Primary lymphoid tissues: where lymphocytes mature; in humans, this is the bone marrow
• Secondary lymphoid tissues: (e.g., lymph nodes; spleen); designed to facilitate antigen presentation to
  mature lymphocytes (interactions between antigen‐ presenting cells [especially DCs] and T or B cells)

Question 3

secondary lymphoid tissues; promote what, examples

Answer 3

• Promote interactions between APCs and T or B cells
• Examples include:
  ‐lymph nodes
  ‐spleen
  ‐Peyer’s patches (intestine)
  ‐bone marrow
  (functions as both a primary and secondary lymphoid tissue)

Question 4

change in clinical practice; tonsils

Answer 4

• Once it was understood that the tonsils are secondary lymphoid tissues that promote immune responses against pathogens in the throat, tonsilectomies became less frequent
• Typical current practice: only remove if pain is severe and/or tonsil size impairs normal function; usually have to suffer multiple substantial episodes of tonsilitis within a given
  timeframe

Question 5

lymphatic drainage

Answer 5

-lymph is the fliud that drains tissue
-major lymph nodes; lots of them
-each lymph node samples antigens from a defined anatomical region

Question 6

lymph nodes

Answer 6

• Afferent lymphatic vessels bring lymph into the nodes
• Efferent lymphatic vessels allow cells to exit the nodes
• Afferent = antigen enters lymph node
• Efferent = activated effector
  cells leave the lymph node to seek out targets

Question 7

where do B cells get activated

Answer 7

-germinal centers
(these are called B cell follicles; the
germinal centre of a B cell follicle is where B cell responses are initiated)

Question 8

lymph node; T cell activation

Answer 8

• T cells activation occurs in the cortex but outside the B cell follicles

-in the cortex of lymph node, but outside of B cell follicle

Question 9

trafficking of lymphocytes

Answer 9

• Whether lymphocytes enter lymphoid vs. non‐lymphoid tissues depends on which markers they express

Example:
*Endothelial cells in lymphoid tissues can express CD34, which is a receptor for CD62L
*Naïve lymphocytes express CD62L, therefore, they get retained in lymph nodes
*Effector cells are CD62L‐, so they pass through the LNs (but look for pathogens in the LNs)

Question 10

where are most lymphocytes distributed

Answer 10

lymph nodes (40%)

Question 11

mitogen; what is it, how does it work, result in what, common examples

Answer 11

• A molecule that induces mitosis (cell division)
• In the immune system, mitogens non‐specifically activate B and T cells, causing them to produce antibodies (B cells) and cytokines (both B and T cells)
• Bypasses the normal requirement for B and T cell receptor‐ mediated recognition of an antigen
• Can result in activation of abnormally high numbers of lymphocytes
• Common examples include the plant‐derived lectins phytohemagglutinin (mainly T cells); concanavalin A (T cells) and pokeweed mitogen (both B and T cells)

Question 12

the third lymphocyte; natural killer cells

Answer 12

• a third major population of lymphocytes that are distinct from B cells and T cells
• play key roles in the innate immune responses to some infections and cancers
• can kill virus‐infected target cells, tumour cells, stressed cells, and some bacteria without prior activation (unlike T and B lymphocytes)

Question 13

NK cells; two ways to kill

Answer 13

• MHC class I molecules are a dominant inhibitory signal for NK cells
• If MHC expression is reduced on a cell, NK cells will be more prone to kill it
• Reducing MHC expression is a strategy used by viruses and tumours to avoid recognition by T cells; NK cells can provide protection in these scenarios
• Alternatively, NK cells may be activated by the expression of stress‐related proteins on target cells

Question 14

NK cells; receptors

Answer 14

• Ly49 (called “Killer Inhibitory Receptors” [KIRs] in some species)
  recognizes MHC class I molecules and suppresses NK cytotoxicity
• NKG2D is a receptor for molecules such as MICA and MICB, which are commonly expressed on stressed cells
• CD16 binds immunoglobulins and triggers target cell death by antibody‐dependent cellular cytotoxicity (ADCC)

Question 15

IFN-gamma; a potent NK cell stimulant

Answer 15

• Interferon gamma potently stimulates both NK cells and macrophages
• Lowers the activation threshold required to trigger NK cell‐mediated killing
• Activated T cells are a source
• Once activated, both NK cells and macrophages can become sources of substantial amounts

Question 16

NK cell memory

Answer 16

• NK cells used to be exclusively categorized as components of the innate immune system because they fit the traditional characteristics, including responding rapidly
• However, they have recently been shown to possess a characteristic akin to “memory”, a hallmark of the adaptive immune system
• NK cells exposed to viruses can adopt epigenetic changes that allow them to respond more effectively against future viral infections

Question 17

innate lymphoid cells 1, 2, 3

Answer 17

• There are three subsets of innate lymphoid cells (ILCs)
• They are designated ILC1, ILC2 and ILC3
• They are often referred to as non NK innate lymphoid cells to clearly differentiate them from the much more common NK cells
• These are rare cells
• They are the greatest ‘pound‐for‐pound’ producers of cytokines
• So, small in # but can play big roles in regulating immune responses
• Therefore, they are implicated in many diseases, either in causing
  or resolving them
• This is an understudied area of immunology

Question 18

Natural Killer T (NKT) cells

Answer 18

• These are innate lymphocytes that express markers characteristic of both NK and T cells
• They use an invariant T cell receptor to respond to antigens presented by a molecule called CD1d (a specialized MHC class I molecule)
• NKT cells can respond more rapidly than conventional T cells
• They are activated by glycolipid antigens (conventional T cells only respond to peptides)