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4. Types of Antigen-Antibody Reactions:
1.Precipitation Reaction:
1. Formation of insoluble Ag-Ab complex.
2. Occurs in liquid or gel media.
2.Agglutination Reaction:
1. Clustering or agglutination of particles.
2. Includes slide, tube, and passive agglutination
tests.
3.Complement Fixation:
1. Involves complement system proteins.
2. Ag-Ab complex fixes complement, leading to
lysis.
4.Immunofluorescence:
1. Use of fluorescent dyes coupled to antibodies.
2. Detection of antigens by emitted visible light.
ELISA (Enzyme-Linked Immunosorbent Assay):
1. Enzyme-labeled Ags and Abs used as serological
reagents.
2. Three types: Indirect, Sandwich, Competitive
ELISA.
3. Indirect ELISA:
1. Coating of microtiter plates with envelope
proteins.
2. Detects HIV antibodies in suspect serum.
4. Sandwich ELISA:
1. Determines Ag presence in a sample.
2. Measures Ag concentration through color
intensity.
5. Competitive ELISA:
1. Variant for estimating antigen concentrations.
2. Utilizes competition between antibody and
sample antigen.

6. Immunologic Principles in
Kidney Transplantation

7. Intro
 Immunosuppression in Kidney Transplantation:
 Maintains allograft function in nonidentical individuals.
 Prevents immune system rejection of the transplanted kidney.
 Immunologic Principles:
 Immune system protects against nonself antigens.
 Innate responses involve phagocytosis and cytokine release.
 APCs activate adaptive immunity by presenting nonself peptides.
 Adaptive Immunity Activation:
 B lymphocytes capture foreign antigens, leading to antibody production.
 T lymphocytes are activated, initiating direct cellular killing.
 Allorecognition and Allograft Rejection:
 Recipient's T and B cells with diverse receptors respond to foreign antigens.
 Mismatched donor HLAs trigger an immune response.
 Allorecognition leads to rejection, impacting graft parenchyma and vasculature.
 Chapter Overview:
 Focuses on immunologic principles in kidney transplantation.
 Covers innate and adaptive immune systems, HLA's role, and allorecognition's pathophysiology.
 Essential for comprehending challenges and mechanisms in kidney transplantation.

9. Innate Immune System Adaptive Immune System
Activation: Activation:
- Triggered by foreign antigens - Follows innate system activation
- Rapid response with no memory - Involves B and T lymphocytes, memory clones
for a quicker response
Components: Components:
- Cellular: Neutrophils, macrophages, - B and T lymphocytes, APCs, immunoglobulins,
DCs, NK cells cytokines
- Molecular: PRRs, complement proteins,
cytokines
Recognition of Antigens: Recognition of Antigens:
- PAMRP and DAMPs recognition by PRRs - B cells bind soluble antigens through BCR
- APCs deliver foreign antigens to SLOs for
presentation to naive T cells
Response to Insult: Response to Insult:
- Cytokine release, opsonization, and - Development of antigen-specific memory
direct killing clones in SLOs for rapid response
- Toll-like receptors (TLRs) recognize - Effector T cells respond with cellular
microbial epitopes killing (CD8+) and cytokine release (CD4+)
- Complement system activation - B cells produce immunoglobulins and
cytokines
Transplantation Relevance: Transplantation Relevance:
- Ischemia/reperfusion injury (IRI) - Allograft rejection due to recognition of
triggers DAMPs release and PRRs donor's mismatched antigens
activation - Memory clones enhance response during
- Impact on delayed graft function repetitive exposures to the same antigen

10. CELLS OF THE IMMUNE SYSTEM
T Lymphocytes:
•Origin: Multilineage bone marrow precursors
migrate to the thymus during T-cell ontogeny.
•TCR Genes: Undergo rearrangement in the
thymus for a diverse T-cell repertoire.
•Positive Selection: Affinity to self-MHC
molecules ensures effective interaction with
self-MHC molecules.
•Negative Selection: Deletion of T cells with
excessively high affinity for self-peptide and
MHC to prevent autoimmune potential.
•Mature T Cells: CD4 or CD8 T cells with
specific TCRs exit the thymus.
T-Cell Stimulation:
•Mediated by APCs: Through receptor-ligand
interactions and cytokine signaling.
•TCRs: Recognize a composite ligand of a
peptide bound to an MHC molecule.
•Signal 1 (Antigen-specific signal): Requires
Signal 2 (Costimulatory signals) for activation.
•Costimulation: Serves as a checkpoint to
prevent self-reactive T cells activation.
•Signal 3: Cytokine secretion (e.g., IL-2) is
crucial for effective T-cell stimulation and
proliferation.

11. Memory T Cell Formation:
•Naive T cells
transformation: Occurs in
secondary lymph organs.
•Effector memory T cells:
Specialize for quick entry
into inflamed tissues.
•Central memory T cells:
Likely remain in secondary
lymph organs.
CD4 T Cells:
•Effector and regulatory
functions: Achieved through
cytokine secretion.
•Subsets: Th1, Th2, Th17, and
Tfh produce specific cytokines.
•Th1 dominance: Associated
with acute cellular rejection.
•Th2 contribution: Noted in
antibody-mediated rejection.
•Regulatory T cells (Tregs):
Maintain peripheral tolerance via
inhibitory cytokines.
CD8 T Cells:
•Cytotoxic T lymphocytes
(CTLs): Specialized in direct
cellular killing functions.
•Mechanisms of killing:
Involves perforin and granzyme
B release, Fas/FasL
interactions.
•Cytokine release: IFN-γ,
TNF-α, TNF-β with direct
cytotoxic effects.
•CD4 CTLs: Presence
identified with similar cytolytic
features to CD8 CTLs.

12. B Lymphocytes:
•Role: Protect extracellular spaces through antibody
production.
•Development: Initiates in primary lymphoid organs (fetal
liver and bone marrow).
•B Cell Receptor (BCR): Contains antigen-specific IgM
and/or IgD, Ig-α, and -β proteins.
•Autoreactive B cells: Undergo receptor editing or
negative selection in primary lymphoid organs.
•Maturation and Differentiation:
• Naive B cells migrate to secondary lymph organs
(SLOs).
• Majority die; remaining mature into memory cells
or plasma cells.
• Class switching in SLOs produces IgA, IgE, IgG, in
addition to IgM and IgD.
• Somatic hypermutation increases BCR affinity to
antigens.
•Plasma cells: Generated in germinal centers with T
helper cell presence for long-lived humoral immunity.

13. Functions of B-Lymphocytes:
•Antibody Production
•Antigen Presentation to T Cells
•Cytokine Production
Surface Markers:
•CD-19 and CD-20 expressed from pro-B cell to memory
B cell.
•CD-38 replaces CD-20 in plasma cells.
•Rituximab targets CD-20, excluding primitive B cells and
plasma cells.
Effector Functions Details:
•Immunoglobulins (Ig) Functions: Complement fixation,
opsonization, cell lysis, and eosinophil degranulation.
•Major Antibody Classes: IgM, IgD, IgG, IgA, IgE.
•IgG: Most abundant, Y-shaped structure, variable (V) and
constant (C) regions.
•Hypervariable Segments: In V regions, determine
antigen specificity through complementarity-determining
regions (CDRs).

14. •Polymorphic MHC Genes:
• Located on chromosome 6, high polymorphism.
• Divided into Class I (HLA-A, -B, -C), Class II (HLA-DP, -DQ, -DR), and Class III.
•Class I MHC:
• Detects intracellular proteins.
• Includes HLA-A, -B, -C.
• Recognized by CD8 T cells, causing target cell destruction.
•Class II MHC:
• Samples extracellular proteins via APCs.
• Includes HLA-DP, -DQ, -DR.
• Presented to CD4 T helper cells.
•Expression during Transplant Rejection:
• IFN-γ induces sustained inflammation, leading to MHC expression.
• Both Class I and II MHC expressed on allograft cells.
•Cross-Presentation:
• Some APCs perform cross-presentation, presenting extracellular antigens on Class I MHC to
CD8 T cells.
• Used for immunity against tumors and viruses.
.
MAJOR HISTOCOMPATIBILITY COMPLEX AND ALLORECOGNITION

16. •Allorecognition Pathways in Transplantation:
• Direct Pathway:
• Donor APCs present own peptides, especially
MHC, to recipient T cells.
• Active early and later in transplantation.
• Indirect Pathway:
• Recipient APCs present donor antigens to
recipient T cells.
• Physiological mechanism of foreign antigen
recognition.
• Semidirect Pathway:
• Transfer of intact donor MHC:peptide
complexes to recipient APCs.
• Involves cross-dressing or cell nibbling.

17. Human Leukocyte Antigen (HLA) Typing and Transplantation:
•MHC class I and II genes are highly polymorphic, resulting in thousands of HLA types.
•Mismatched donor HLAs are seen as foreign antigens, leading to allograft rejection.
HLA Typing Techniques:
•Originally performed using sera from multiparous women or individuals with blood transfusions.
•Current methods involve molecular biology techniques like PCR sequencing or next-generation sequencing.
•Complete allele matching reduces the risk of sensitization but may limit transplant opportunities.
Commonly Matched HLA Alleles:
•HLA-A, -B, and -DR used in HLA-matching algorithms, but practices may vary between centers.
Anti-HLA Antibody Screening:
•Patient screened for preexisting anti-HLA antibodies (DSA) associated with rejection and poor graft survival.
•Single antigen bead assay is the preferred method for DSA testing.
Virtual Crossmatch:
•Determines the presence of anti-HLA antibodies against a specific donor HLA antigen.
•Helps exclude potential recipients with a positive cross-match without performing the actual assay.

18. Non–Major Histocompatibility Antigens:
•Minor histocompatibility antigens, polymorphic proteins within a species, cause rejection despite MHC
matching.
•Examples:
• H-Y antigen from the Y chromosome.
• MICA antigens, involved in innate immunity.
• ABO blood group glycolipids and angiotensin-1 receptors.

19. Allograft Rejection:
•Types: T-cell–mediated or cellular rejection (TCMR), Antibody-mediated rejection (AMR).
•Involves T cells, B cells, plasma cells, macrophages, NK cells, and neutrophils.
•Endothelial and tubular cells are especially affected.
Acute T-Cell–Mediated Rejection (TCMR):
•Phagocytosis of donor antigens by APCs.
•Migration into SLOs, activation of T cell clones, and migration into the allograft.
•Core features: interstitial inflammation, tubulitis, and, in rare cases, endarteritis.
•T cells, especially Th1, release cytokines affecting alloimmune response.
Acute Antibody-Mediated Rejection (AMR):
•Develops due to preexisting antibodies or de novo antibody formation (DSAs).
•Binding mediates graft damage through various mechanisms.
•C4d deposition indicates complement-fixing antibodies.
•Involves NK cells, neutrophils, monocytes, leading to microvascular inflammation.
•Diagnostic criteria include histologic evidence, C4d staining, and evidence of circulating antibodies.
Chronic Rejection:
•Late allograft dysfunction due to alloimmune and non-alloimmune mechanisms.
•Chronic active AMR: transplant glomerulopathy, peritubular capillaropathy, arteriopathy.
•Chronic active TCMR: interstitial fibrosis, tubulitis, or allograft arteriopathy.

20. Transplantation Tolerance:
•State with no destructive immune response towards a well-functioning donor allograft.
•Achieved through control of T-cell reactivity by central and peripheral mechanisms.
Central Tolerance:
•Thymic deletional mechanisms eliminate T cells reactive against donor antigens.
•Experimental models use irradiation and donor hematopoietic progenitor cell infusion to induce chimerism.
•Clinical translation with concurrent bone marrow and kidney transplantation shows promising outcomes.
Peripheral Tolerance Mechanisms:
•Include deletion, anergy, and regulation.
•Tregs (regulatory T cells) crucial for peripheral tolerance.
Strategies under Investigation:
•Costimulatory blockade.
•Pharmacologic manipulation of DCs (dendritic cells).
•Induction of Tregs.
•IL-2 receptor stimulation and ex vivo expanded Tregs show promise in clinical studies.
Challenges and Progress:
•Lack of reproducible immune monitoring assays for tolerance.
•Ongoing progress in developing clinical tolerance strategies.
•Transplantation tolerance not yet a clinical reality, but advancements are being made.