Introduction to Heamatopoiesis II Flashcards
How do we study haemic cells?
The way we do this is by flow cytometry and that’s sometimes referred to as fluorescence activated cell sorting (FACS).
Describe flow cytometry
So with flow cytometry, you measure your cells and the fluorescence and then lose the cells.
FACS can actually use the different markers on cells to actually sort cells into collection tubes depending on the markers.
We use a monoclonal antibody which recognises the antigen on the surface of a specific haemic cell to label the cell. The antibody will bind to the antigen and then a second antibody which is fluorescently labelled will bind to the stem structure of the 1st antibody to create a tower of antibodies. The cells are passed through a laser which excites the fluorescent antibody which is then detected by a flow cytometer or another type of spectrophotometric instrument.
CD antigens
CD - Cluster of differentiation
A way of naming the different antigens present on the surface of cells. We use specific antibodies to bind to CD antigens to detect types of haemic cells e.g. lymphocytes or T helper Cells (CD4).
Changes in surface antigen expression mark B cell differentiation.
What can the immune system be divided into?
Leukocytes are cells that form a major part of the immune system and can be divided into two arms: the innate and adaptive immune system.
Innate Immune System
Innate is very fast but non-specific. Adaptive immune system is slow acting but is specific and it can generate immune memory. Within the innate immune system, we have phagocytes (neutrophils, basophils and eosinophils) which can consume foreign particles along with monocytes which can migrate from the blood to tissue to become tissue-resident cells. We also have platelets which are just fragments of cells that play an important role in blood clotting but also an important role in immunity because they can opsonise or create toxic environments for invading pathogens.
What cells act between the innate and adaptive immune system?
Natural killer cells are cells between innate and adaptive immune system. So they are innate immune cells, but they are lymphocytes and they are cells which are able to kill virally infected cells as part of their immune surveillance role.
Adaptive Immune System
We then have adaptive immune cells e.g. T and B lymphocytes and these are there to provide increased specificity of recognition of pathogens and generation of memory B cells.
Neutrophils
Short-lived, motile cells designed to phagocytose bacteria and combat tissue infection
Basophils
Only circulating leukocytes that contain histamine, activated by antigen cross linking of FcꜪRI and receptor-bound lgE.
Eosinophils
Host defence against nematodes and other parasitic infections.
Platelets
Activated by antigen crosslinking of FcyRII
What are the two main lymphocytes?
T cells and B cells
T cells
The T cells are involved in Cell mediated immunity and can differentiate into T helper cells (CD4+) which are involved in release of cytokines that are involved in directing the immune response and helping other B cells to create antibodies. T cells can also differentiate into T cytotoxic cells (CD8+) and these are directly involved in killing virally infected cells.
B cells
The B cells are involved in humoral immunity (production of antibodies and immune memory) through the differentiation into plasma cells.
Antigen
An antigen is a molecule, could be a protein or sugar or parts of the bacteria cell wall, that can initiate an immune response.
Epitope
A section of amino acids in a 3D structure that is recognised by the lymphocyte receptor is known as the epitope.
What do B lymphocyte receptors recognise?
B lymphocyte receptors recognise soluble antigens.
What do T cell receptors recognise?
T cells receptors recognise antigens “presented” to the T cell - they need to have the antigen presented to them on a molecule called MHC class one that could be on a surface of a virally infected cell or a dendritic cell (antigen presenting cell).
How do T cells and B cells create diversity within the antigen binding region?
Genetic recombination is used which takes place very early in B and T cell development.
What do pro T cells and pro B cells do in the thymus to initiate genetic recombination?
So the pro B cells in bone marrow or pro T cells within the thymus will activate expression of the genes that code for the B and T cell receptor respectively and once it does that, it’s committed on its line to differentiate into that specific T-cell or B cell once it starts expressing the receptor gene.
What other gene is also expressed along with the receptor genes of pro B and T cells?
This is expressed along another gene called RAG gene (recombinase activating gene) and this acts on segments on DNA that code for the antigen binding region of B and T cells.
What segments of DNA are present on the antigen binding region DNA of receptors
Within these antigen binding regions of these receptors, you have segments of DNA that code for either a variable region (V segment), a diversity segment (D segment) or a joining segment (J segment). Many possibilities between the V, D and J segment.
What doea the RAG (recombinase activating gene) do?
What happens is the RAG enzyme catalyses genetic recombination, such that the cell will randomly take some V segments, some D segments and some J segments and put them together to form the antigen binding region of the receptor. This will create a receptor that has a pocket that is unique in terms of recognising an epitope on a particular antigen. This is random within each cell that passes through this process.
Estimated that there are more than 10^10 combinations of V, D and J segments. The immune system therefore has the necessary diversity to recognise the wide range of pathogens it comes in to contact with.
What happens in the lymphoid system?
Major site of B and T cell interaction with antigen-presenting cells.
Mature naïve B and T cells migrate here to become activated.
Interaction with cognate T helper cells allow adaptive immunity to occur
B cells differentiate:
Undergo SHM ( somatic hypermutation) and class switch to produce high affinity functional antibodies
To long-lived memory cells
To plasma cells
Cd8+ T cells:
Effector (cytotoxic) T cells
Memory T cells
Major organs of the lymphoid system
Lymph node and spleen
Lymph node
Lymph nodes is where interaction between B and T cells encounter antigen presented cells takes place. These antigen presenting cells pick up and process foreign material, infectious material. Process it into antigens and migrate into the lymph nodes in order to present antigens to lymphocytes.
Spleen
The spleen is diverse, divided into two areas, the white and red pulp. The red pulp is responsible for control of red blood cell integrity (recycling of red blood cells and platelets). The white pulp is responsible for the immune function and the organisation is similar to that of the lymph node.
How are germinal centres formed?
B cells in lymph nodes are found clustered together as follicular B cells in lymphoid follicles and when activated by an antigen, these lymphoid follicles become germinal centres.
B cell Maturation
Germinal centre reaction:
B cells enter lymph node to encounter antigen and help from CD4+ T cells
Affinity maturation occurs to enhance Antibody-antigen interaction.
Generation of immune memory: Memory B cells and plasma cells.
Immunoglobulin Molecule
The basic structure:
Heavy and light chains, encoded by different genes
Constant region functions to bind specific receptors (Fc receptors)
Variable region functions to bind to antigen (determined by VDJ recombination)
Immunoglobulin Class Switch
Immunoglobulin class switch requires another genetic recombination event. The same event that mediates SHM (Activation Induced Cytidine Deaminase - AID) also mediates class switch from lgM/lgD to: lgG (1, 2a/b, 3), lgA and lgE
Types of Immunoglobulin molecules
IgM, IgD, IgG, IgA and IgE
Class switch of immunoglobulin molecules
IgM/IgD to: IgG (1, 2a/g, 3), IgA and IgE
IgM
Activation of serum complement
IgG
Opsonisation/activation of complement/immune cell activation
IgA
Innate Immune cell activation
IgE
Innate Immune Cell Activation
Defence Against Viral Attack
CD8+ T cells engage with infected cells through interaction with MHC class I presentation of viral peptides.
Immune memory is generated with the help of CD4+ T cells
Antigen-presenting cells interact with both CD4+ and CD8+ T cells, CD4+ T cells help with APCs to interact with CD8+ T cells to promote their differentiation to memory T cells.
