IMSE311 Immunology and Serology: Study Notes

Historical Development of Immunology

• 1500s: Ancient Chinese variolation
• 1798: Edward Jenner — Smallpox vaccination; Cross-immunity
• 1880: Louis Pasteur — Live, attenuated vaccines (chicken cholera, anthrax)
• 1885: Therapeutic vaccination; first live attenuated vaccine for rabies
• 1897: Robert Kaus — Discovered precipitins
• 1901: Emil von Behring — Serum; antitoxin
• 1905: Robert Koch — Cellular immunity in TB
• 1908: Elie Metchnikoff — Phagocytosis; Paul Ehrlich — Immunity
• 1913: Charles Richet — Anaphylaxis
• 1919: Jules Bordet — Nature of complements
• 1930: Karl Landsteiner — Human blood antigens
• 1960: Burnet & Medawar — Immunologic tolerance; Nobel Prize in Immunology
• 1972: Gerald Edelman, Rodney Porter — Structure of antibody
• 1977: Rosalyn Yallow — Radioimmunoassay
• 1980: Snell, Dausset, Benaceraf — Major Histocompatibility Complex (MHC)
• 1984: Jerne, Koehler, Milstein — Immunoregulation; Monoclonal antibodies
• 1991: Murray, Thomas, Doherty, Zinkernagel — Transplantation; T cell recognition of virally infected cells
• 1996: Doherty, Zinkernagel — Cytotoxic T cell recognition of virally infected cells
• 2008: Barre-Sinoussi, Montagnier — HIV Nobel Prize

Terminology

• Immunology: Study of host’s reaction when foreign substances are introduced into the body
• Immunity: Condition of being resistant to infection
• Antigens: Foreign substances that induce a host response
• Antibodies: Serum proteins produced by certain lymphocytes when exposed to a foreign substance and they react specifically with that foreign substance (antigen)
• Immunogen: Any agent capable of inducing an immune response
• Antigen: Any agent capable of binding to components of the immune response (lymphocytes, antibodies)
• Epitope (antigenic determinant): portion that binds specifically with the binding site of an antibody or a lymphocyte; typically 4-6 amino acids or 5-7 monosaccharides
• Paratope: antigen-binding site of an antibody
• Hapten: incomplete antigen that needs a carrier to become immunogenic
• Carrier: protein that provides the size/complexity needed for hapten immunogenicity

Immunogenicity and Antigen Properties

• Immunogenicity requirements depend on multiple factors:
  • Molecular size: Immunogenic ≥ $10{,}000$ Da; Active immunogens > $100{,}000$ Da
  • Foreignness: greater taxonomic distance increases immunogenicity; examples: Autologous, Syngeneic, Allogeneic, Heterologous, Heterophilic
  • Complexity: structural complexity enhances immunogenicity
  • Genetic composition: host genetics influence response (MHC and other genes)
• Hapten-carrier concept (Landsteiner): hapten alone is not immunogenic; conjugation to a carrier protein creates a complete antigen
• Antigenic determinants (epitopes) consist of amino acids or carbohydrate moieties; surface exposure determines accessibility

Antigens and Immunogens; Immunogenicity Determinants

• Antigen vs Immunogen: Immunogen may be antigenic but not all antigens are immunogenic
• Major antigens can be on RBCs, WBCs, platelets; exposure routes include transfusion, tissue grafts, transplacental leakage, ingestion/injection
• Cross-reactivity and molecular mimicry occur when immune responses to one antigen react with a structurally similar second antigen; can cause autoimmunity
• Molecular mimicry examples include shared epitopes between microbial antigens and host proteins

Major Histocompatibility Complex (MHC)

• MHC molecules determine histocompatibility of transplanted tissue; present endogenous (Class I) and exogenous (Class II) antigens to T cells
• Found on all nucleated cells (Class I) and antigen-presenting cells (Class II); Class III encodes various immune components (e.g., complement proteins)
• Class I genes: HLA-A, HLA-B, HLA-C; present endogenous peptides to CD8+ T cells
• Class II genes: HLA-DP, HLA-DQ, HLA-DR; present exogenous peptides to CD4+ T cells
• Class III region encodes components like C2, C4A, C4B, and TNF family proteins
• TAP1/TAP2 proteins transport peptides into the endoplasmic reticulum for Class I loading
• Location: chromosome 6; MHC class I presents to CD8+, class II presents to CD4+

Transplantation Immunology

• Immunogenic tissues (most to least): Bone marrow, skin, islets of Langerhans, heart, kidney, liver, bone; cornea is least immunogenic
• Graft rejection timing and mechanisms:
  • Hyperacute: within minutes; primarily humoral (antibody-mediated)
  • Accelerated: 2–5 days; cell-mediated components
  • Acute: 7–21 days; predominantly cell-mediated
  • Chronic: >3 months; cell-mediated with ongoing inflammation

Adjuvants in Immunization

• Adjuvants enhance the immune response to an immunogen
• Types licensed for humans: Alum (aluminum hydroxide/phosphate); MF59, AS03
• Freund’s complete adjuvant is used experimentally (mineral oil, emulsifier, killed mycobacteria)
• Mechanism: adjuvants promote antigen presentation and costimulation (e.g., via CD40L, B7) to enhance both humoral and cell-mediated responses

Vaccines: Types and Examples

• Live, attenuated: Measles, Mumps, Rubella (MMR); Varicella; Yellow Fever; Rotavirus; Oral polio; BCG (tuberculosis)
• Inactivated: Salk vaccine (polio); Hepatitis A; Rabies
• Subunit vaccines: Toxoids (diphtheria, tetanus); Polysaccharide (pneumococcal, meningococcal); Purified protein vaccines (e.g., acellular pertussis); Recombinant vaccines (HBV, HPV, some COVID-19 vaccines)
• Subunit vaccine examples and notes shown in study materials

Humoral and Cellular Immunity; Line of Defense

• Acquired (Adaptive) Immunity is split into two parts:
  • Cell-mediated (T-cell) immunity: cytotoxic T cells kill infected cells; also involved in graft rejection and some hypersensitivity
  • Antibody-mediated (humoral) immunity: B cells produce antibodies; neutralize toxins, opsonize pathogens, activate complement
• Timeline of primary vs secondary responses:
  • Primary response: latent period ~ $5-15$ days; IgM appears first
  • Secondary response: rapid anamnestic response; IgG dominates; occurs within $2-4$ days
• Natural vs artificial activation: Natural active (infection), Artificial active (vaccination); Passive immunity (transfer of antibodies) is short-lived

Lymphoid System and Lymphoid Organs

• Lymphoid system: The key immune cells are lymphocytes (20–40% of circulating WBCs); typical lymphocyte size ~ $7-10 extmu m$
• Primary lymphoid organs: Bone marrow (B cell development) and Thymus (T cell development)
• Secondary lymphoid organs: Spleen (antibody synthesis; red/white pulp), Lymph nodes (cortex, paracortex, medulla), Mucosa-associated lymphoid tissue (MALT), Cutaneous-associated lymphoid tissue (CALT), Tonsils, Appendix
• Lymphocyte development and recirculation: lymphocytes develop in primary organs, migrate to secondary organs, recirculate between tissues
• The spleen includes red pulp (filters aging/damaged RBCs) and white pulp (periarteriolar lymphoid sheaths, germinal centers)
• B and T cell localization:
  • T cells: paracortical regions of lymph nodes, PALS in spleen, thymus-derived
  • B cells: follicles in cortex of lymph nodes, germinal centers, marginal zone of spleen

T Lymphocytes (T Cells)

• About 60–80% of circulating lymphocytes are T cells; mature in the thymus
• T cell precursors (thymocytes) enter thymus; differentiation occurs in cortex and medulla
• Markers:
  • CD3, CD4 (helper T cells), CD8 (cytotoxic T cells)
  • CD2, CD5 (general T cell markers)
• Coreceptors and TCR complex:
  • CD3/TCR complex with αβ or γδ chains
  • Positive selection in cortex ensures functional TCR; negative selection eliminates strongly self-reactive T cells
• T cell subtypes and functions:
  • Th1: IFN-γ, TNF-β; activate macrophages; promote cellular immunity
  • Th2: IL-4, IL-5, IL-10, IL-13; promote B cell activation and humoral immunity; support eosinophils
  • Tc (CD8+): cytotoxic activity against infected/tumor cells via perforin/granzymes
  • T Regulatory (Treg): CD4+CD25+; suppress immune responses to self-antigens; produce TGF-β and IL-10
  • T helper 2 (Th2) vs Th1 balance influences infection vs allergy outcomes
• T cell development stages:
  • Double-negative (CD4− CD8−; γδ or αβ TCR rearrangement)
  • Double-positive (CD4+ CD8+; TCR rearrangement; CD3/TCR expression)
  • Positive selection → single-positive (CD4+ or CD8+)
  • Negative selection to remove self-reactive cells

B Lymphocytes (B Cells)

• B cells develop in bone marrow; mature B cells express surface Ig (IgM and IgD)
• B cell development stages: Pro-B → Pre-B → Immature B → Mature B → Activated B
• Surface markers: CD19, CD20, CD21; CD40; MHC class II
• B cell fate after activation: plasma cells (antibody-secreting) and memory B cells
• Subsets and localization: marginal zone B cells, follicular B cells
• Differentiation in bone marrow requires rearrangement of heavy chain on chromosome 14 and light chains on chromosomes 2 (κ) and 22 (λ)

Surface Markers and Lymphocyte Typing

• Common markers and cell types:
  • CD2, CD3, CD4, CD8 (T cells)
  • CD19, CD20, CD21 (B cells)
  • CD56, CD94, CD16 (NK cells)
  • CD10 (pre-B cells); CD34 (stem cells in hematopoiesis; not shown here)
• Isotypes and receptor classes help identify lymphocyte subsets and activation status

Immunoglobulins (Antibodies)

• Major classes and features:
  • IgG: $150{,}000 ext{ Da}$; 75–80% of serum Ig; monomer; crosses placenta; subclasses IgG1–IgG4
  • IgM: $900{,}000 ext{ Da}$; pentamer with 10 binding sites; first antibody produced in primary response; efficient agglutination; poor placental transfer
  • IgA: $160{,}000 ext{ Da}$; found in serum and mucosal secretions; forms secretory IgA with J chain and secretory component; activates alternative pathway
  • IgD: $180{,}000 ext{ Da}$; low serum abundance; acts as B cell receptor
  • IgE: $190{,}000 ext{ Da}$; low serum concentration; binds mast cells/basophils; mediates allergic responses and anti-parasite defense
• Structural features:
  • Light chains: κ (on chromosome 2) and λ (on chromosome 22); two types of light chains; variable and constant regions
  • Heavy chains: γ (IgG), μ (IgM), α (IgA), δ (IgD), ε (IgE); heavy chain constant regions determine isotype
  • Disulfide bonds: interchain and intrachain; hinge region provides flexibility
  • Antibody monomer is composed of two identical heavy and two identical light chains (Y-shaped)
• Fragmentation and functional regions:
  • Fab (antigen-binding) and Fc (constant) regions; Papain digestion yields Fab + Fab + Fc; Pepsin digestion yields F(ab')2 + Fc
• Functions of immunoglobulins:
  • Neutralization of toxins, opsonization, complement activation (classical pathway), agglutination, precipitation, and antibody-dependent cellular cytotoxicity (ADCC)
• Immunoglobulin variability:
  • Isotypes (heavy chain classes): IgG, IgA, IgM, IgD, IgE
  • Allotypes (allelic variations in constant regions)
  • Idiotypes (varying antigen-binding paratopes)

Immunoglobulin Genetics and Variability

• Isotypic variation: heavy chains define isotypes (IgG, IgA, IgM, IgD, IgE)
• Allotypic variation: differences in constant regions among individuals
• Idiotypic variation: differences in variable (paratope) regions

Immunoglobulin-Specific Details

• IgG: predominant in serum; has subclasses IgG1–IgG4; crosses placenta; diff abilities to fix complement
• IgM: pentamer; strong agglutination; activates complement strongly; mostly intravascular
• IgA: serum IgA1; secretory IgA (SIgA) with J chain; secretory component; mucosal immunity
• IgD: primarily B cell receptor; function not fully defined
• IgE: allergic responses; parasites; binds mast cells and basophils; minimal serum
• Serum protein electrophoresis shows gamma globulin region corresponding to IgG

Antigen Structure and Immunogenicity: Hapten-Carrier and Determinants

• Antigenic determinants (epitopes) are the specific parts of an antigen that bind antibodies or TCRs
• Antibody-binding region: Fab; Antigenic determinants often involve 4–6 amino acids or 5–7 monosaccharides
• Hapten-carrier concept explains how small molecules become immunogenic when conjugated to a larger carrier protein
• Major histocompatibility complex (MHC) genetic influence on immunogenicity and transplant compatibility

Major Immunological Concepts in Immunology

• CROSS-REACTIVITY and MOLECULAR MIMICRY:
  • Cross-reactivity occurs when antibodies or TCRs recognize similar epitopes on different antigens
  • Molecular mimicry can lead to autoimmune disease when immune responses to pathogens react with host tissues
• ADJUVANTS and VACCINES:
  • Adjuvants enhance immune response by improving antigen presentation and stimulating innate immunity
• GRAFT REJECTION AND COMPATIBILITY:
  • MHC matching reduces risk of rejection; HLA typing is clinically important for transplantation, paternity studies, and genetic counseling

Pattern Recognition Receptors (PRRs) and Innate Immunity

• PRRs: encoded by host genome; sensors for extracellular infection; recognize PAMPs (pathogen-associated molecular patterns)
• Toll-like receptors (TLRs): membrane-spanning glycoproteins with leucine-rich repeats (LRRs)
  • High density on monocytes, macrophages, neutrophils
  • Cell surface TLRs: TLR1, TLR2, TLR4, TLR5, TLR6
  • Endosomal TLRs: TLR3, TLR7, TLR8, TLR9
• Specific TLR-ligand associations:
  • TLR1: Mycobacteria
  • TLR2: Gram-positive bacteria; peptidoglycan/teichoic acid
  • TLR3: Endosomal dsRNA
  • TLR4: Gram-negative LPS
  • TLR5: Flagellin
  • TLR6: Mycoplasma
  • TLR7/8: Viral ssRNA
  • TLR9: Bacterial/viral DNA
  • TLR10: Unknown
• Other PRRs:
  • C-type lectin receptors (CLRs): bind mannans and β-glucans in fungal walls
  • RLRs (RIG-I-like receptors): recognize viral RNA in cytoplasm, induce type I IFNs
  • NOD-like receptors (NODs): detect peptidoglycans; mutations linked to Crohn’s disease

Phagocytosis and the 2nd Line of Defense

• Phagocytosis: engulfment and digestion of pathogens by leukocytes; key steps (7 steps):
  1) Adherence: contact between phagocyte and microbe via opsonins
  2) Engulfment: cytoplasm extends around microbe
  3) Phagosome formation: microbe enclosed in phagosome
  4) Granule contact: lysosomal granules fuse with phagosome
  5) Phagolysosome formation: hydrolytic enzymes released into phagosome
  6) Digestion: enzymatic breakdown of microbe
  7) Excretion: contents expelled by exocytosis
• Inflammation and phagocyte activation enhance this process

Neutrophils

• Most abundant circulating leukocytes (50–75% of WBCs); 10–15 μm; multi-lobed nucleus
• Granules: primary (azurophilic) containing myeloperoxidase, lysozyme, elastase, proteinase-3, cathepsin G, defensins; secondary granules containing lysozyme, lactoferrin, collagenase, gelatinase; respiratory burst components (NADPH oxidase)
• Primary function: phagocytosis and destruction of microbes; secondary functions include NET formation
• NETs: Neutrophil extracellular traps trap and kill microbes when neutrophils die

Eosinophils; Basophils; Monocytes/Macrophages; Mast Cells; NK Cells

• Eosinophils: 1–3% of WBCs; bilobed; combat parasites; involved in allergy; regulate mast cells; contain MBP and other granule contents
• Basophils: <1% of WBCs; histamine-containing granules; promote immediate hypersensitivity; assist in B cell IgE production
• Monocytes: 4–10% of WBCs; migrate to tissues to become macrophages; phagocytic; produce IL-1 and IL-6; markers include peroxidase, arylsulfatase, lysozyme, lipase
• Macrophages: tissue-resident phagocytes; innate killing and antigen presentation (to T cells); secrete IL-1, IL-6; express C3b receptors; multiple tissue-specific names (Kupffer cells, alveolar macrophages, microglia, Langerhans cells, splenic macrophages, osteoclasts, etc.)
• Mast cells: similar to basophils; granules contain histamine and proteases; bind IgE; important in barrier defense and APC functions; located in mucosal surfaces
• Natural Killer (NK) cells: 5–10% of circulating lymphocytes; CD16+, CD56+; cytotoxic to virally infected and tumor cells; can perform AB-dependent cellular cytotoxicity (ADCC) via CD16; relatively MHC-unrestricted; activated by IL-2

Acute-Phase Reactants and Inflammation

• Acute-phase reactants: liver-produced proteins that rise during infection/inflammation; regulated by cytokines IL-1, IL-6, TNF-α
• Key acute-phase proteins:
  • C-reactive protein (CRP): marker of inflammation; opsonization; complement activation; main substrate: phosphocholine; half-life ~ $18 ext{ hours}$; rises within $4-6$ hours; peaks within 48 hours; hs-CRP used for cardiovascular risk (0.01 mg/L baseline)
  • Serum amyloid A (SAA): rises in infection; correlates with HDL; higher in bacterial infections; peaks 24–48 hours
  • Complement components (C3, C4, etc.): mediators of inflammation; opsonization and lysis; C3 rises ~ 48–72 hours (~2x)
  • Alpha-1 antitrypsin: protease inhibitor; regulates cytokines; deficiency linked to emphysema and cirrhosis
  • Haptoglobin: binds free hemoglobin; antioxidant; increases during inflammation
  • Fibrinogen: coagulation factor; crosses endothelium to aid wound healing; increases with inflammation
  • Ceruloplasmin: copper transport; oxidizes iron; increases during inflammation
• Inflammation: systemic response with 5 cardinal signs (rubor, tumor, calor, dolor, functio laesa); stages: vascular, cellular, proliferation/repair
• Vascular response: vasodilation; increased permeability; extravasation of proteins like fibrinogen and complements; redness, warmth, swelling, pain
• Cellular response: migration of neutrophils first; macrophages later; phagocytosis and cytokine production
• Resolution/repair: fibroblast-driven tissue repair; possible abscess or granuloma formation depending on context

The Innate vs Adaptive Immunity: Two Main Arms

• Innate (Natural) Immunity: non-specific; rapid; barriers, phagocytes, NK cells, mast cells; first two lines of defense
• Adaptive (Acquired) Immunity: specific; develops after exposure; memory; involves T and B lymphocytes; third line of defense
• Natural vs Adaptive: Innate is non-specific but immediate; Adaptive is specific with memory and clonal expansion
• Key terms: Cell-mediated immunity (T cells) vs Humoral immunity (B cells/antibodies)

Lymphoid System and Lymphoid Organs, Repertoire, and Development

• Lymphocytes: primary cells of the adaptive immune response; mature in thymus (T cells) and bone marrow (B cells)
• Primary lymphoid organs:
  • Bone marrow: B cell development; main source of hematopoietic stem cells
  • Thymus: T cell development; thymic cortex and medulla provide selection processes
• Secondary lymphoid organs: spleen, lymph nodes, MALT, CALT, tonsils, appendix; sites of antigen encounter and lymphocyte activation
• The spleen structure:
  • Red pulp: filters aged red cells
  • White pulp: consists of PALS (periarteriolar lymphoid sheath) and lymphoid follicles with germinal centers
• Lymph node organization:
  • Cortex: B cell zones (follicles)
  • Paracortex: T cell zones (HD region)
  • Medulla: plasma cells and macrophages

Antigen-Presenting Cells and T Cell Activation

• APCs include macrophages, dendritic cells, B cells
• Process: antigen uptake, processing, presentation on MHC molecules, T cell activation via TCR recognition and co-stimulation (e.g., CD28-B7 interactions)
• Key T cell types activated by APCs: CD4+ helper T cells (MHC II) and CD8+ cytotoxic T cells (MHC I)

T-Cell Activation and Function

• Activation markers: IL-2 receptor (CD25) upregulation; clonal expansion of activated T cells
• Effector functions:
  • Th1: macrophage activation; cytotoxic T cell responses; IFN-γ production
  • Th2: B cell help; antibody production; IL-4, IL-5, IL-10, IL-13
  • Tc: direct cytotoxicity via perforin/granzymes; ADCC with NK cells
• Regulatory T cells (Tregs): suppress immune responses to self; maintain tolerance; cytokines include TGF-β, IL-10

B-Cell Activation and Differentiation

• Antigen encounter in primary follicles triggers activation
• Activation markers: CD25 (IL-2 receptor); formation of blast cells; differentiation into plasma cells and memory B cells
• B-cell development sequence: Pro-B → Pre-B → Immature B → Mature B (IgM+IgD) → Activated B
• Important surface markers: CD19, CD20, CD21; MHC class II; CD40

Antigen Recognition: CD Markers and Lymphocyte Typing

• T cells: CD3+, CD4+, CD8+; major subsets include Th (CD4+), Tc (CD8+)
• B cells: CD19+, CD20+, CD21+; memory B cells express various markers
• NK cells: CD16+, CD56+; cytotoxic without antigen exposure; respond to IL-2

Immunoglobulins: Structure, Classes, and Function (in-depth)

• Immunoglobulins are the humoral component of immunity; produced by plasma cells
• Major classes and characteristics (summary):
  • IgG: $M = 150{,}000 ext{ Da}$; 75–80% of serum Ig; IgG subclasses IgG1–IgG4; crosses placenta; opsonization; complement fixation
  • IgM: $M = 900{,}000 ext{ Da}$; pentamer with 10 antigen-binding sites; strong agglutination; first responder in primary response; does not cross placenta
  • IgA: $M = 160{,}000 ext{ Da}$; serum IgA and secretory IgA (SIgA) in mucosal secretions; J chain; secretory component protects from proteolysis; activates alternative pathway
  • IgD: $M = 180{,}000 ext{ Da}$; low serum; B cell receptor
  • IgE: $M = 190{,}000 ext{ Da}$; low serum; binds mast cells and basophils; immediate hypersensitivity and anti-parasite defense
• Structural features:
  • Light chains: κ (chromosome 2) and λ (chromosome 22)
  • Heavy chains: γ (IgG), μ (IgM), α (IgA), δ (IgD), ε (IgE)
  • Disulfide bonds; hinge region
• Protein domains and regions:
  • Variable regions (VH, VL) for antigen binding
  • Constant regions (CH1, CH2, CH3, CH4) determine class and effector function
  • Fc region interacts with complement and Fc receptors
• Fragmentation:
  • Papain digestion yields Fab + Fab + Fc
  • Pepsin digestion yields F(ab')2 + Fc
• Immunoglobulin features:
  • Avidity and affinity differences; cross-reactivity potential; class-switch recombination under cytokine influence (e.g., IL-4 promotes IgG/IgE switching; IFN-γ promotes IgG1/IgG3)
• Immunoglobulin genetic variability:
  • Isotypes (heavy chain classes), Allotypes (constant region variants), Idiotypes (variable region diversity)

Antibody Production Theories (Historical)

• Ehrlich’s Side Chain Theory: receptors for antigen exist pre-exposure; antigen selects receptor-bearing cells; receptors released into circulation as antibodies; new receptors formed
• The Template Theory: antigen molds antibody synthesis; antibody produced to fit antigen shape
• Clonal Selection Theory: each lymphocyte is pre-programmed for a single antibody; antigen selects and expands these cells to generate a clonal population

Immunoassays and Immune Testing

• ELISpot: enzyme-linked immunospot assay; used to measure cytokine secretion at the single-cell level
• ELISA: basic immunoassay used to detect antibodies or antigens in samples

Cytokines: Mediators of Immunity

• Cytokines are small soluble proteins that regulate the immune response; actions can be autocrine, paracrine, or endocrine
• Major families: TNF, Interleukins (IL-1, IL-2, IL-4, IL-6, IL-10, IL-12, IL-13, IL-17, etc.), IFN, TGF-β, CSF
• Innate immunity cytokines:
  • IL-1: endogenous pyrogen; promotes diapedesis; stimulates acute-phase proteins; fever
  • TNF-α: vasodilation, increased vascular permeability; enhances neutrophil function; promotes inflammation
  • IL-6: drives acute-phase response; stimulates B and T cells; produced by many cell types
  • IFN-α/β: antiviral; activate NK cells; upregulate MHC class I
• Adaptive immunity cytokines:
  • IL-2: T cell growth factor; promotes T cell proliferation; activates NK cells; supports Th1/Th2 differentiation
  • IL-4: promotes Th2 differentiation; enhances B cell activation; supports IgG and IgE production; upregulates MHC I/II
  • IL-10: anti-inflammatory; suppresses antigen presentation; promotes regulatory functions
  • IL-12: from dendritic cells/macrophages; drives Th1 differentiation; enhances NK activity
  • IFN-γ: key Th1 cytokine; activates macrophages; enhances antigen presentation via MHC I/II; upregulates many immune genes
  • TGF-β: anti-inflammatory; regulates cell growth and differentiation; maintains tolerance; modulates CD8/CD4 cells
• Chemokines: subfamily of cytokines; guide leukocyte migration; classes include CXC, CC, CX3C, and C
• Receptors for cytokines: IL-2R, IL-6Rα, gp130; signaling pathways lead to transcriptional changes and immune activation

Primary and Secondary Lymphoid Organs: Summary

• Primary lymphoid organs:
  • Bone marrow: B cell development; stem cell origination; hematopoietic niche
  • Thymus: T cell development; cortex and medulla provide selection signals
• Secondary lymphoid organs:
  • Spleen: red pulp and white pulp; filters blood and mounts immune responses to blood-borne antigens
  • Lymph nodes: cortex (B cells), paracortex (T cells), medulla (plasma cells); sites of antigen presentation and adaptive immune activation
  • MALT (GALT, BALT, NALT), CALT, TONSILS, APPENDIX; mucosal immunity
• Lymphocyte recirculation: movement between blood and lymphoid tissues to surveil for antigens

T-Cell Development: Thymus Maturation

• Thymocytes originate as CD4−CD8− cells; rearrange TCR genes; differentiate into CD4+CD8+ (double-positive) cells
• Positive selection in cortex ensures TCRs recognize self-MHC with appropriate affinity
• Negative selection in medulla deletes strongly self-reactive T cells; results in single-positive CD4+ or CD8+ mature T cells
• Mature T cells exit thymus and traffic to peripheral lymphoid tissues
• CD markers important for T cell identification: CD2, CD3, CD4, CD8
• Typical CD4+/CD8+ ratio in healthy individuals ~$2:1$; HIV infection often reduces CD4+ count relative to CD8+

B-Cell Development: Bone Marrow Origins

• B cells undergo heavy chain rearrangement on chromosome 14 (Ig heavy chain); light chains (kappa on chromosome 2, lambda on chromosome 22)
• Stages: Pro-B cell → Pre-B cell → Immature B cell → Mature B cell
• Surface markers evolve from CD19, CD45 (early) to IgM and IgD on mature B cells
• Mature B cells populate spleen and lymph nodes; upon activation, differentiate into plasma cells and memory B cells

Table: Lymphocyte Characteristics (Summary)

• Helper T cells (Th): CD3+, CD4+, CD8−; function: stimulate B cells and macrophages; cytokine production; principal markers: CD2, CD3, CD4
• Cytotoxic T cells (Tc): CD3+, CD4−, CD8+; function: lysis of virus-infected and tumor cells; CD16/Fc receptors enable ADCC in some contexts
• NK cells: CD16+, CD56+; function: innate cytotoxicity against infected/tumor cells; NK cells bridge innate and adaptive responses
• B cells: CD19+, CD20+, CD21+; function: antibody production; humoral immunity

Antigen Presenting Cells and Antigen Processing

• APCs: dendritic cells, macrophages, B cells
• Presentation pathways:
  • Endogenous antigens via MHC Class I to CD8+ T cells
  • Exogenous antigens via MHC Class II to CD4+ T cells
• Co-stimulation is required for full T cell activation (e.g., CD28-B7 interactions)

Major Immunological Concepts in Transplantation

• Allorecognition and histocompatibility are driven by MHC (HLA) differences
• Graft rejection and tolerance depend on MHC compatibility and co-stimulatory signals
• Immunosuppressive strategies aim to reduce T cell activation and cytokine signaling to prevent rejection

Major Immunologic Properties of Antibodies and Antigens

• Antibodies neutralize pathogens, opsonize for phagocytosis, and activate the complement system
• Antigenic determinants (epitopes) are recognized by antibodies or TCRs
• Antigen-presenting cells present peptide/MHC complexes to T cells; B cells recognize native antigens via surface immunoglobulin

Appendix: Common Nomenclature and Notes

• SRBC: Sheep red blood cells, used in rosette formation experiments for T cell identification
• Rosette formation: used to identify T cells by binding to sheep RBCs
• CD markers are standardized reagents used to differentiate lymphocyte subsets
• Spleen’s white pulp is organized around PALS and lymphoid follicles; red pulp performs filtration of aged erythrocytes
• Primary immune responses produce a lag before detectable antibodies; secondary responses rely on memory B and T cells and are faster and more robust
• MHC Class I presents endogenous peptides to CD8+ T cells; MHC Class II presents exogenous peptides to CD4+ T cells
• The immune response is influenced by age, health status, dose, route of administration, and genetic background

References to Study Materials

• Refer to the Clinical Immunology and Serology: A Laboratory Perspective (5th edition) by Christine Dorresteyn Stevens for specific figures, tables, and diagrams referenced in the lecture transcript