Specific+Defences

=Specific Defences= This component of the immune system is centred around the immune response of lymphocytes. T-cells and B-cells respond to specific antigens. T – cells respond via cell mediated immunity.

Cell-mediated immunity is an immune response that does not involve antibodies, but rather it involves antigen-specific cytotoxic T-lymphocytes B –cells are used via antibody mediated immunity aka humoral immunity in that it uses an antibody (NB/ an antigen is a substance or part of a substance that activates the immune system.)

There are a number of properties of this type of immune response.
 * Specific - it is activated by an antigen (Ag) and the response is for that Ag only.
 * Versatile – there are lots of possible Ag’s and our immune system can distinguish between them
 * Memory – it remembers which Ag have been previously encountered therefore response after 2nd Ag exposure is faster, stronger and longer than response to 1st exposure. Since the body has already made the antibodies it is easy to reproduce them and not have to “start from scratch”.
 * Tolerance - some types of Ag’s the immune system doesn’t respond to which is good as it can tell the difference between self and non-self

Antigen Presenting Cell (APC)

 * Antigen-presenting cell is a macrophage which digests a foreign cell, but leaves the antigens intact.
 * It then binds these antigens to a Major Histocompatibility Complex (MHC) molecule on its cell membrane.
 * Our cells are dotted with protein molecules (self-antigens) that are not antigenic to us but are strongly antigenic to others [[image:ucanaandp/Antigen_presentation.jpg width="370" height="327" align="right"]]
 * One type, MHC proteins, mark a cell as self. These are glycoproteins which are made in the cytoplasm and then inserted into the cell membrane.
 * Each MHC molecule has a deep groove that displays a peptide, which is a normal cellular product of protein recycling
 * During their move in infected cells they collect Ag fragments and insert them into this groove
 * The two classes of MHC proteins are:The antigen-MHC complexes are noticed by certain lymphocytes (recognition) which promotes cell division (repeated cell divisions).
 * Class I MHC proteins – found on virtually all body cells
 * Class II MHC proteins – found on certain cells in the immune response e.g. APC membranes and lymphocyte membranes

T-Cells - Helper T-cells and Cytotoxic T-cells[[image:ucanaandp/T cell activation.jpg align="right"]]

 * T cells arise from stem cells in the bone marrow -
 * They travel to the thymus where the differentiate and mature.
 * At maturity, they acquire receptors for self markers (MHC molecules) and for antigen-specific receptors.
 * They are then released into the blood as "virgin" T cells.

T cells ignore other cells with MHC molecules and they ignore free-floating antigens. However, they will bind with a antigen-presenting macrophage (a macrophage possessing a MHC-antigen complex).Some recognise Class I MHC + AG other Class II MHC + Ag. Which cell recognises Class I and Class II MHC depends on cluster designation (CD) markers on the T-cell.
 * CD8 cells recognise Class I MHC’s e.g. cytotoxic and suppressor T cells are both CD8 T cells and they are activated via cell mediated immune response.
 * CD4 cells recognise Class II MHC’s and these are helper T –cell and are activated by Antibody (Ab)-mediated immune repsonse (help B cells)

Therefore activated CD8 can be one of three types:


 * 1) Tc - cytotoxic T-cells - bind Class I MHC-Ag on membrane target cells and kills these cells
 * 2) Tcm - memory cytotoxic T-cells - not immediately active in response to Ag. On reappearance of Ag ‘2nd challenge’ are immediately converted to Tc cells
 * 3) Ts – Suppressor T-cells - appear after Tc’s produced and act to suppress Tc action via secretion of supression factors

B-cells
Immature B-cells have a range of Ab’s on their surface. When Ag binds to an Ab B-cell is activated or sensitised. It is then cloned so that there are heaps of Antibodies

Ag also binds to Class II MHC on B-cell surface

Helper T-cells bind Ag with Class II MHC on B-cell which activated B-cell to differentiate into Plasma and memory B-cells

Plasma cells produce and secrete Abs for specific Ag

Memory B-cells respond to alter infections or exposures by same Ag

Antibody So what are antibodies any way? Antibodies are also called Immunoglobulins. Antibodies responding to different antigens have different V regions but the C region is the same for all antibodies in a given class
 * Constitute the gamma globulin portion of blood proteins
 * Are soluble proteins secreted by activated B cells and plasma cells in response to an antigen
 * Are capable of binding specifically with that antigen[[image:ucanaandp/Antibody.jpg align="right"]]
 * Consists of four looping polypeptide chains linked together with disulfide bonds
 * Two identical heavy (H) chains and two identical light (L) chains
 * The four chains bound together form an antibody monomer
 * Each chain has a variable (V) region at one end and a constant (C) region at the other
 * Variable regions of the heavy and light chains combine to form the antigen-binding site
 * C regions form the stem of the Y-shaped antibody and:
 * Determine the class of the antibody
 * Serve common functions in all antibodies
 * Dictate the cells and chemicals that the antibody can bind to
 * Determine how the antibody class will function in elimination of antigens

There are five classes of antibodies: IgD, IgM, IgG, IgA, and IgE IgD – monomer attached to the surface of B cells, important in B cell activation IgM – pentamer released by plasma cells during the primary immune response IgG – monomer that is the most abundant and diverse antibody in primary and secondary response; crosses the placenta and confers passive immunity

IgA – dimer that helps prevent attachment of pathogens to epithelial cell surfaces

IgE – monomer that binds to mast cells and basophils, causing histamine release when activated

Innate Defences (part 2) Immune System - content Review Questions Moodle Assessment