Hypersensitivity

1. PA-9
Immunity & Hypersensitivity reactions

2. Natural or innate immunity
• non-specific and is considered as the first line of defense without
antigenic specificity.
• It refers to intrinsic mechanisms that are poised to react immediately,
and thus constitute the first line of defense.
• It is mediated by cells and molecules that recognize products of
microbes and dead cells and induce rapid protective host reactions.

3. Components
It has 2 major components :
• a) Humoral: comprised by complement.
• b) Cellular: consists of neutrophils, macrophages, and natural killer
(NK) cells.

4. • Epithelial cells-Act as mechanical barrier to entry of microbes from
external environment.
• It produce defensins & lymphocytes located in epithelial cell.
• Monocytes & Neutrophils-enter the tissue and sense the presence of
microbes & other offending agents-ingest & destroy them.
• Dendritic cells-capture protein antigens & display peptides for
recognition by lymphocytes. They have rich collection of receptors
that sense microbes & cell damage and stimulate secretion of
cytokines

5. Mechanism of Innate immunity
• The innate immune system provides host defense by two main reactions.
• Inflammation- Cytokines and products of complement activation, as well as
other mediators, are produced during innate immune reactions and trigger the
vascular and cellular components of inflammation.
The recruited leukocytes destroy microbes and ingest and eliminate damaged
cells. The innate immune response also triggers the repair of damaged tissues.
• Antiviral defense- Type I interferons produced in response to viruses act on
infected and uninfected cells and activate enzymes that degrade viral nucleic
acids and inhibit viral replication, inducing what has been called an antiviral
state.
NK cells recognize virus-infected cells.

6. Adaptive immunity
• Definition: characterised by antigenic specificity.
2 main components:
• a) Humoral: consisting of antibodies formed by B cells.
It which protects against extracellular microbes and their toxins.
It is mediated by B (bone marrow–derived) lymphocytes and their
secreted products, antibodies (also called immunoglobulins, Ig).
• b) Cellular: It is mediated by T (thymus-derived) lymphocytes
Responsible for defense against intracellular microbes and against
cancers

7. Cells of Immune system

8. Role in immunity
• Epithelial cells-Act as mechanical barrier to entry of microbes from
external environment.
• It produce defensins & lymphocytes located in epithelial cell.
• Monocytes & Neutrophils-enter the tissue and sense the presence of
microbes & other offending agents-ingest & destroy them.
• Dendritic cells-capture protein antigens & display peptides for
recognition by lymphocytes.
• They have rich collection of receptors that sense microbes & cell
damage and stimulate secretion of cytokines

9. Cell and their Function

10. Mechanism of Humoral immunity

11. Mechanism of cell mediated immunity.

12. Major histocompataibility Complex(MHC)
• products of genes that evoke rejection of transplanted organs, and their
name derives from their role in determining tissue compatibility
between individuals.
• function of MHC molecule: is to display peptide fragments of protein
antigens for recognition by antigenspecific T cells.

13. HLA complex and the structure of HLA

14. Types of MHC
• Depending upon the characteristics of MHC, they have been divided into 3
classes :
• Class I MHC antigens - loci as HLA-A, HLA-B and HLA-C.
CD8+ lymphocytes carry receptors for class I MHC and these cells are
used to identify class I antigen on them.
• Class II MHC antigens -single locus as HLA-D.
These antigens have further 3 loci: DR, DQ and DP.
identified by B cells and CD4+ cells.
• Class III MHC antigens -components of the complement system (C2 and
C4) coded on HLA complex but are not associated with HLA expression
Not used in antigen identification.

15. Mechanism of MHC
• 1. Different antigenic peptides bind to different MHC molecules-
It follows that an individual mounts an immune response against a
protein antigen only if he or she inherits an MHC variant that can bind
peptides derived from the antigen and present it to T cells.
• 2. By segregating cytoplasmic and internalized antigens-
MHC ensure that the correct immune response is mounted against
different microbes—CTL-mediated killing of cells harboring
cytoplasmic microbes and tumor antigens, and helper T cell–mediated
antibody production and macrophage activation to combat extracellular
and phagocytosed microbes.

16. Hypersensitivity reaction
• Also called as -IMMUNOLOGIC TISSUE INJURY.
• Defined as an exaggerated or inappropriate state of normal immune
response with onset of adverse effects on the body.
• The lesions of hypersensitivity are a form of antigen antibody reaction.
• These lesions are termed as hypersensitivity reactions or immunologic
tissue injury, of which 4 types are described: type I, II, III and IV.

17. Classification
• on the basis of the underlying immunologic mechanism

18. Mechanism of type I hypersensitivity reactions

19. Examples of type 1

20. Mechanisms of antibody-mediated injury-II

21. Examples of Antibody-Mediated Diseases-II

22. Immune complex disease-TYPEIII

23. Examples of Type III

24. Mechanisms of T-cell–mediated (type IV)

25. Examples of Type IV

26. Case scenario-1
• After giving a intracutaneous injection of antigen in a previously
immunized animal, resulted in localized area of tissue necrosis.
• What is the type of hypersensitivity reaction.?
• What is Mechanism of injury?

27. Case scenario-2
• A person was attacked by honeybee who landed in emergency in
shock & respiratory distress.
• What is pathology?
• What type of hypersensitivity reactions?

28. Case scenario- 3
• A 45 year old female c/o joint pain for 1 year. On Investigations,
inflammation & destruction of articular cartilage seen.
• What is type of Hypersensitivity reaction?
• Mechanism of Tissue injury?

29. Case scenario -4
• A person presented with Complains of multiple joint pain, multiple
subcutaneous nodules ,palpitations and fever. Had history of sore
throat 4-6 weeks back.
• What type of hypersensitivity reactions?
• What is target antigen in this disease.?

30. Questions
• PA 9.1 Describe the principles and mechanisms involved in immunity.
• 9.1.1. Define innate immunity.
• 9.1.2.Describe the components and mechanism of innate immunity.
• 9.1.3. Define and enumerate the types of Adaptive immunity.
• 9.1.4. Describe the cells of the immune system and their role in immunity.
• 9.1.5. Describe the mechanism of Humoral immunity
• 9.1.6. Describe the mechanism of cell mediated immunity.
• 9.1.7. Define and describe the mechanism of Major histocompataibility
Complex(MHC).

31. • 9.2.1.Define and classify Hypersensitivity reactions.
• 9.2.2.Describe the mechanism of type I hypersensitivity reactions with schematic
diagram with examples.
• 9.2.3. Describe the mechanism of type II hypersensitivity reactions with
schematic diagram with examples.
• 9.2.4.Describe the mechanism of type III hypersensitivity reactions with
schematic diagram with examples.
• 9.2.5. Describe the mechanism of type IV hypersensitivity reactions with
schematic diagram with examples.
• 9.2.6.Categorise the given clinical scenario into different types of
hypersensitivity reactions.

32. THANK YOU