Intro to the haematopoietic system_ part 2.pdf

20/10/23 Intro to haematopoietic system: part 2 Tools to study haemic cells: Every cell uses a unique combo of antigens. Each antigen can be potentially recognised by an antibody. Flow cytometry is a way of measuring markers on cells using uorescence ( uorescent antibodies ). Antibody q fluorescent...

20/10/23 Intro to haematopoietic system: part 2 Tools to study haemic cells: Every cell uses a unique combo of antigens. Each antigen can be potentially recognised by an antibody. Flow cytometry is a way of measuring markers on cells using uorescence ( uorescent antibodies ). Antibody q fluorescent marker p Antigen I excited Igians A ow cytometer is a particle counter. It uses a light source in through fork of a laser and then this laser emits light which can be detected by a photo multiplier tube as an event and recorded by a computer. The ow cytometer has a uid in them called sheath uid, which helps the cells to ow through the instrument essentially in single le. Events are detected as voltage pulses. The length of time the laser is disrupted ( cell in front of it ) is proportional to the size of the cell. Markers of cell differentiation: 1. CD antigens = Cluster of Differentiation Changes in surface antigen expression mark B cell differentiation. Innate vs Adaptive immune system: Lymphocyte receptors recognise antigens. Lymphocyte receptors: Recognise recognise antigen antigens soluble to presente the t cell I The epitope is the section of the antigen that binds to the cell. T-cells cannot recognise soluble antigens. Creating diversity within the antigen binding regions of the T-cell receptor and B-cell receptor is achieved by a process of DNA editing and reassortment in order to create those unique sequences. This genetic recombination takes places very early in B/T-cell development. So either pro-B cells in the bone marrow or pro-T cells in the thymus will activate expression of the genes that code for either the B or T cell receptor and once it’s done that, it’s committed on its line to differentiate into that speci c T-cell or B-cell. Once it starts expressing the receptor gene, RAG is also expressed. This stands for recombinanase activase gene. What this enzyme does it is acts on segments of the chromosome that code for the antigen binding region of the B-cell of T-cell. Within these antigen binding regions, there are segments of DNA that code for either a variable region ( V segment ), a diversity region ( D segment ) or a joining segment ( J segment). The RAG enzyme catalyses genetic recombination, such that the cell will randomly take some random V-segments, some D-segments, some J-segments and put them together to form the antigen binding region of the receptor. This is random within each cell so creates lots of different epitopes. Organs of the lymphoid system: Antigen g Major site of B and T cell interaction with APCs. presenting cons Mature naive B cells and T cells migrate here to become activated Interaction with cognate T helper cells allows adaptive immunity to occur B cells differentiate: undergo SHM and class switch to produce high af nity functional Abs, to long lived memory cells, to plasma cells CD8+ T cells: effector cytoxic T-cells and memory T cells. I Antibodies Lymph nodes and the spleen: The major organs of the lymphoid system of the lymph nodes and the spleen. This is where all these interactions take place. Lymph nodes are the major sites where B and T-cells encounter antigen presenting cells. These presenting cells pick up and process foreign material and infectious material, process it into antigens and then migrate to the lymph nodes in order to present them to the B or Tcells. B-cells are commonly found together as follicular B-cells and when activated these lymphoid follicles become germinal centres. The spleen is divided into t wo distinct areas, the white and the red pulp. The red pulp is responsible for the control of red blood cell integrity ( recycling red blood cells when they become old ). The white pulp is responsible for the immune function of the spleen and the organisation of the white pulp of the spleen is similar to that in lymph nodes. B-cell maturation: Germinal centre reaction: B-cells enter the lymph node to encounter antigen and help from CD4+ T-cells Af nity maturation occurs to enhance Ab-Ag interaction Generation of immune memory ( memory B cells and plasma cells ) The germinal centre is where the B-cells develop their antibody response and differentiate. This happens through gene mutations called somatic hyper mutations. When the B-cells encounter antigen, they move to the border of the T-cell zone where they encounter an activated CD4+ T-cell that also recognises that antigen. This T-cell provides growth and differentiation signals back to the B cell so it can express CD40 ligand on it’s surface to interact with CD40 receptor on B-cells. T cells can also produce cytokines which can do various things to the B-cell. For example, stimulate them to produce memory cells or the T cell can stimulate the B cell to enter the follicle where it undergoes rapid proliferation to form a germinal centre within the dark zone. Within this environment, somatic hypermutation within that region, codes for antigen binding in the B-cell receptor. The enzyme that mediates somatic hypermutation is called activation induced cytosine deaminases. Somatic hypermutation changes the af nity of the antigen binding site such that the cell moves from the dark zone where it undergoes proliferation and somatic mutation to the light zone. Once in the light zone, it encounters antigen presenting follicular dendritic cells which bear the antigen that the B-cell receptor recognised as well as antigen speci c CD4+ T-cells. Once here, the whole system will test whether or not somatic mutation has changed the binding site. So in those B-cells where somatic hypermutation has led to increased af nity for the antigen, those cells will sur vive. If there has been a decrease af nity it will die. This process continues until you have B cells expression the higher af nity for their antigen. And at that point, the AID enzyme catalyses a class switch recombination to switch from producing IgM to producing other immunoglobulin subtypes such as IgG, IgA and IgE. This change of af nity is called af nity maturation. Building adaptive Ab immunity: There is a lag phase of 4 to 5 days so that clinal expansion of B-cells can take place. This is why you feel ill within this period. Within this time, short lived plasma blasts and plasma cells are produced in order to allow IgM speci c to the antigen is generated. At the same time, the germinal centre reaction takes place, which takes longer, it produces high af nity B-cells which produce complementary antibodies. These antibodies will have undergone a class switch to create functionality in terms of how they respond to ght the infection. Am immunoglobulin is the same as an antibody. The immunoglobulin molecule: Heavy and light chains encoded by different genes Constant region functions to bind speci c receptors Variable region functions to bind antigen Immunoglobulin class switch: Class switch requires another genetic combination event. The same enzyme that mediates SHM ( activated induced cytidine deaminase ) also mediates class switch from IgM/IgD to: IgG IgA IGE Defence against viral attack: CD4 vs CD8 T-cells CD4 in conjunction with the TCR recognises antigens in context with MHC class II CD8 in conjunction with the TCR recognises antigens in context with MHC class I