Cell and Molecular Biology of the Immune System

-Induced by exposure to an infection -Shows a high degree of specificity -Exhibits ‘memory’

-Not overreact to benign or self -Direct different effector mechanisms against different pathogens -Recognise and respond to any invading organism

-Mediated by lymphocytes (B/T cells) -Clonally distributed receptors -Large repertoire - low freq. of cells specific for any antigen -Response takes time to develop, memory cells produced

• Innate response • Adaptive response

-B cell receptor - antibody -Membrane form of immunoglobulin binds ‘free’ antigen -Subsequently secreted when B cell is activated now known as antibody

-T cell receptor -a chains and B chain -membrane form only -doesn’t bind free antibody, recognises peptide fragment of antigen bound to MHC expressed by APC

-Basic 4 chain structure - 2 heavy chains and 2 light chains held together by covalent and non-covalent bonds. -2 types of L chains (lambda and kappa) -Each chain has a variable and constant region -Bivalent

5 - IgM, D, A, G and E Called isotypes - determined by heavy chain

3 gene segments (V, D and J - V is biggest)

2 gene segments (V and J)

After DNA breaks, a single V and a single J gene segment are joined together to encode the V region of the light chain. This process is random - there are two loci containing L chain genes: lambda and kappa). Similarly, a single random V, D and a J gene segment are joined together in a single B cell to encode the V region of the heavy chain Occur in the same B cell to end up with an antibody with randomly generated chain regions.

-H chain gene segments rearrange (D-J, then V-DJ)(greater variability in H chain as V, D and J segments) -Light chain gene segments rearrange - kappa segments (V-J) first -If this is unsuccessful, then lambda segments (V-J) rearrange

-Ig gene segment rearrangement is guided by special sequences flanking each of the V, D and J gene segments = recombination signal sequences -Rearrangement involves a complex of enzymes = V(D)J recombinase -(recombination activating gene) -RAG-1 and RAG-2 genes encode lymphoid-specific components of the recombinase -Mutations in RAG genes result in immunodeficiency

-In each B cell, only 1 rearranged H chains gene from one chromosome is expressed. -Similarly, only one rearranged L chain from one chromosome is expressed by each individual -Light chain isotope exclusion - each B cell expresses either a rearranged kappa or lambda light chain - never both. -These mechanisms ensure that each individual B cell produces one randomly generated antibody.

• Multiple gene segments for each chain. • Combinatorial diversity - different V,D, J segments recombine to produce different sequences • Combinations of heavy and light chains • Junctional diversity increases diversity further - imprecise joining, N regions • Somatic hypermutation (SHM)

-Performed by the enzyme, activation-induced deaminase (AID) -AID acts on DNA to deaminate cytosine to uracil -Uracil is then recognised by error-prone DNA repair pathways leading to mutations

IgM - mu IgD - delta IgG - gamma IgA - alpha IgE - epsilon

-Following Ag recognition as each B cell differentiates, it will start to secrete its unique BCR as an antibody -The secreted form made by each B cell has an alternative constant region that lacks a transmembrane region. As the original re-arranged VDJ regions are not altered, the secreted antibody has the same antigen specificity as the membrane BCR. -The membrane and secreted forms are produced by alternative RNA processing

-Encoded by a different C region gene segment (i.e. Cm, Cd, Cg, Ce , Ca) -4 gamma chain gene segments correspond to the four IgG subclasses; similarly two a chain gene segments -At the the heavy chain locus, the Cm segment is physically closest to the V, D and J gene segments and so IgM is the first antibody expressed by each developing B cell

-They leave the bone marrow, and their V region genes may undergo SHM in specialized areas of secondary lymph organs following the binding of Ag. and the activation of the B cell -This leads to further BCR diversity being generated -These activated B cells start to secrete their BCR (as an antibody) -Class-switch recombination may then occur, as a result of the cytokines that may be present in this environment

Similar to that seen in BCR: -multiple V,D,J gene segments -combinatorial diversity -junctional diversity Unlike BCR, TCR is never secreted and no SHM occurs in TCR genes

-Allows the binding of many peptides that can be presented to T cells - provides a clear evolutionary advantage to the population as can respond to almost unlimited no. of different pathogens However, -Increases risk of immune-mediated disease -Reduces pool of available donor organs

-Highly polymorphic (diverse at population level) -Individual has a limited number of different forms -Influence which peptides can be presented

1) Ag. (e.g. viral protein) synthesised in cytoplasm 2) protein cleaved to peptides by proteasome 3) peptides transported to endoplasmic reticulum by TAP transporter 4) peptides bind to MHC class I molecules 5) MHC-I/peptide complex then transported to cell surface