The Ultimate Immunology Study Guide Part 2

VIII. . Cells of Immune System I: Phagocytes 13 A. Neutrophils = white blood cell that leads immune system's response ● Myeloid cell lineage ● Efficient in phagocytosis ● Large #s ● 1st type of ces to present a barrier to pathogens ● Perform only 1 phagocytic even & die B. Macrophages ● Large cell ~ 20 µm in diameter ● Perform many phagocytic events (simultaneously) C. Phagocytosis 1. Attachment: ● ZETA potential ● Importance of opsonization ● Ingestion/digestion ● Respiratory burst ● Phagosomes - phagolysosomes 2. Discharge (neutrophils die & degranulate) 3. Antigen presentation by macrophage → Chemotaxis = movement directed towards external chemical stimuli using chemoattractant/repellents IX. Cells of Immune System II: Lymphocytes A. B cells ● Originates from bone marrow ● Pre-B cells have cytoplasmic µ chain only; light chain produced & mIgM appears - immature B cells ● Migrate to secondary lymphoid organs & start to express mIgD + IgM & w/ same antigenic specificity = respond to antigens (mature B cells) ○ Proper stimulation includes interactions w/ specific antigen & subset of T cells => B cells activated ○ Blast trandoramtoin transforms mature B cells into antibody-secreting plasma cells which secrete Ig for a few days & die → Class switching = changes B cell production of Ig from 1 type to another - B. T cells ● ● ● ● ● 14 During process, constant-region of antibody heavy chain changed, but variable region stays same Variable region does not change =doesn't affect antigen specificity Retains affinity for same antigens, but can interact w/ diff. effector molecules Originate from stem cells in bone marrow (prothmocytes) Moves into thymus Immature thymocytes (CD4-CD8-), double negative (DN) Rearrange TCR & become Double Positive Majority of produced thymocytes die due to expressed specificity against self-antigens a. TH2- helps B cells b. TC - cytotoxicity /// TH1- helps TC c. Treg, TH17 - regulates immune system C. Selection of Thymocytes in Thymus 15 X. Complement A. Cascade Rxns ● Strong control of potentially self-damaging rxns ● Inc. amount of steps ● Independently controlled/shut off 1. Complement ○ Serum proteins ○ 10% of total serum globulins ○ Secreted by few types of cells (macrophages, liver, & epithelial) B. Classical Pathways = initiated by antigen-antibody complexes with the antibody isotypes IgG and IgM ● C1 → C4 → C2 → C3 1. C1 Components ○ C1q, C1r, C1s ○ Stoichiometry: C1q(C1r)2(C1s)2 *Each C1s cleave up to 2 hundred C4 (amplification step) 2. Steps i. C1q binds antigen-bound antibody & induced conformational change in C1r activating it ii. C1r activates 2nd C1r & 2 C1s iii. C1s cleaves C4 & C2 iv. C4 cleaved 1st & C4b binds to membrane close to C1 v. C4b binds C2 & exposes it to C1s vi. C1s cleaves C2 crating C3 convertase (C4b2a) vii. C1s cleaves C4 into 2 frag.: large C4b & small C4a (leaves cascade) viii. C1s change conformation & cleaves C2 into 2 parts: small C2b (leaves cascade) & large C2a ix. 16 C3 converstase splits C3 into: smaller C3a (leaves cascade) & larger C3b -- some combine w/ C3 convertase to form C5 convertase *C4b2a -= C3 convertase x. C3b of C5 convertase binds C5 allowing C4b2a to cleave C5 C4b (not enzyme) ○ Hydrophobic - attaches to membrane & specific binding site for C2a ○ Hydrophobic site unstable & active only a fraction second 4. C2a ○ Binding sites: 1 for C4b forming C4b2a & 1 for 2nd binding site is catalytic site for C3 5. C3b production (most important product) ○ Hydrophobic binding sites - attaches to membrane ○ Large quantities of C3b produced - opsonization ○ Neutrophil & macrophages have low affinity (affinity is not enhanced or decreased after antibodies are produced - it remains the same = strength of binding. Whed such antibodies cover the antigens - binding with there Fab fragments, Fc fragments "stick out" , and capture phagocytes' low affinity Fc receptors, triggering phagocytosis) C3b receptors ○ 2nd binding site specific to C4b2a forms C4b2a3b: C5 convertase C. Alternative Pathways = opsonize and kill pathogens; natural defense against infections 1. Steps i. C3 spontaneously/slowly breaks down => producing small quantities of C3b ii. C3b reversibly binds factor B (C3bB) - weak C3 convertase iii. When factor B is in C3bB complex => cleaved by favor D into 2 frag.: Ba (leaves cascade) & Bb (reversibly attached to C3b) *Stability inc. by factor P (properdin)/dec. by factor H *Efficiency of factor H in destabilizing C3bBb inc. by presence of sialic acid (common in mammalian cells) iv. C3bBb dissociates & C3b frag. irreversibly inactivated by factor I (generating C3c & C3d) v. Bacterial cells inhibit factor H thus allowing C3bBb (C3bBbP) to act as alternative C3 convertase *Microorganism opsonized by Abs & complement are consumed by phagocytes w/ Fc & C3b receptors D. Terminal Pathways = prevents the formation and membrane insertion of the MAC by binding the terminal complement proteins (C8 and C9) as they unfold - C4b2a3b - classical C5 convertase // (C3b) 2 BbP 1. Steps i. Cleave C5 into small C5a (leaves cascade) & C5b binds in succession C6 ii. C7 & C8 form C5b678 complex on membrane (causes polymerization of C9 & formation of membrane attack complex (MAC) which destroys membrane) * Both performance and the C9 component of the MAC contain a common 'MACPF' domain and form pores in the cell membrane as part of their function. The MAC targets gram-negative bacteria and certain pathogenic parasites, while perforin, released by natural killer cells or cytotoxic T lymphocytes (CTLs), targets virus-infected and transformed host cells iii. Soluble fragments released during complement activation (chemoattractants for phagocytes come & feast on opsonized microorganism & anaphylatoxins - causing mast cell degrandu 3. XI. ● ● ● ● ● Generation of Antibody Diversity Virtually every Ab contains unique AA seq. in variable region, but only limited # of invariant seq. in constant region In germ-line DNA, multiple gene seg. encode portions of single Ig heavy/light chain Seg. cannot be transcribed & translated into complete chains until rearranged into functional genes During B cell maturation, some gene seg. randomly shuffled by dynamic genetics system generating more 10 6 combinations Processing inc. repertoire over 10 8 combinations ● ● 17 After Ag stimulation of mature B cell in peripheral lymphoid organs => more rearrangement of constant-region gene seg. Generating changes in isotype expressed => producing change in bio. function of Ig w/o chaing specify Mature B cells contain c’some DNA that is not identical to germ-line DNA A. Multigene Organization of Ig Genes ○ κ & λ light chains & heavy chains encoded by separate multigene families on diff. C’somes ○ In germ-line DNA, each of multigene families contain several coding sequences. - gene seg. B. V, J, C rearrangements Light Chain Heavy Chain ○ 2 rearrangements: DJ + VDJ Both Cδ & Cµ transcribed mRNA include either Cδ/Cµ Both IgM & IgD have identical variable regions Rearranged VJ encodes VL Human λ-chain DNA Mouse - 31 V 4J λ 7C λ κ-chain DNA - 40 V κ 5J κ 1C κ Heavy-chain DNA - 51 V H 27 D H 6 J H Series of C H – each encodes a separate constant region domain λ - 3V 4J 4C - 85 V κ 5J κ 1C κ (no subtypes) λ λ λ gene segments (J λ 4 = pseudogene) (3 λ subtypes) C. Variable Region Gene Rearrangement - Produces mature immunocompetent B cells - Each cell committed to produce Ab w/ binding site encoded by particular seq. of rearranged V genes - 18 Rearrangement is seq. of random events resulting in random determination of B cell specificity D. Generation of Antibody Diversity ● Variability derived from junctional flexibility, P-addition, N-addition, & somatic mutations (# of possibilities 10 → P-addition = palindromic seq. repair enzymes’ addition → N-addition = addition of random N-nucleotides by TdT E. Class Switching 10 ) 19 ● Switch regions are located upstream from C H seg except Cδ F. Simultaneous Expression of IgM & IgD ● Primary RNA transcript contains 4 poly-A sites 20