Innate immunity provides the first line of defense against pathogens and involves physical, chemical, and cellular barriers. It responds immediately through pattern recognition receptors on innate immune cells like macrophages, neutrophils, and dendritic cells that recognize pathogen molecular patterns. These receptors activate the cells and initiate responses like phagocytosis, inflammation, and complement activation to help fight infection. Innate immunity provides non-specific protection and acts as the initial response until adaptive immunity can mount an antigen-specific response.

1. INNATE IMMUNITY
ABINAYA MOORTHI

2. Innate immunity
• Non-specific defense mechanism.
• Responds immediately or within minutes.
• Classified into
a) 1st line defense: anatomic ,chemical &
microbiological barriers.
b) 2nd line defense: antimicrobial proteins,
phagocytes and other cells.

3. 1st line defense-Barriers.

4. 2nd line defense.
• Sensor cells: macrophages, neutrophils and
dendritic cells.
• Sensor cells express Pattern Recognition
Receptors(PRR’s)
• PRR’s-discriminate between self and non-self.
-Recognize Pathogen Associated
Molecular Patterns(PAMP’s).
• This activates innate immune cells.

5. Pattern Recognition Receptors[PRR’s]
Classified on basis of their cellular localization
and their function;
a) Free receptors in serum e.g.. ficolins,
histatins
b) Membrane-bound phagocytic receptors.
c) Membrane-bound signaling receptors.
d) Cytoplasmic signaling receptors e.g. NOD like
receptors.

7. Pathogen Associated Molecular
Patterns.
• PAMP’s -are proteins that are part of many
microorganisms but not of the host own body
cells. They include;
i. mannose-rich oligosaccharides, peptoglycans
and lipopolysaccharides of bacterial cell wall.
ii.unmethylated CpG DNA common to many
pathogens

8. Phagocytic cells
• Include macrophages, monocytes, granulocytes
and dendritic cells.
• Ingest pathogens via either phagocytosis,
receptor-mediated endocytosis or
macropinocytosis.
• Have 3 surface receptor types;
C-type lectin family receptors
Scavenger receptors.
Complement receptors.

9. Phagocytes.
• Intra-membranous receptors- G-protein
coupled receptors(GPCR’s).
Stimulate killing of microbes.
Directs responses to anaphylatoxins
Example: Fmet-Leu-Phe(FMLF) receptor that
signals production of reactive oxygen species.

10. Toll-Like Receptors.
• Are single pass trans-membrane proteins.
• Have a Leucin rich repeat(LRR) extracellular domain.
• There are 10 expressed TLR genes hence limited
specificity.
• Activated by PAMP’s of pathogens extracellularly or in
endocytic vesicles.

11. Other key receptors.
• RIG-I-like receptors detect cytoplasmic viral
RNA and activate MAVS.
• NLRP react to infection/cell damage through
an inflammasome to induce cell death and
inflammation.
• Cytosolic DNA sensors signal through STING to
induce production of interferon I.

12. Innate Lymphoid cells.
• Develop from the common lymphocyte
progenitor like B & T cells.
• Include Natural Killer(NK) cells.
• Lack T & B-cell antigen receptor and co-
receptor complex.
• However, they express receptor for IL-7.

14. Complement system.
• Composed of > 30 plasma proteins
synthesized especially in the liver.
• Three pathways so far;
Classical pathway- antibody triggered
pathway.
Alternative pathway- activated by presense of
pathogen alone.
Lectin pathway- activated by lectin-type
proteins.

15. Inflammation.
• Induced by cytokines, chemokines,
prostaglandin, leukotrienes and platelet
activator factor.
• Roles ;
Delivery of additional effector molecules &
cells
Induce local blood clotting.
Promote repair of injured tissues.

16. Inflammation.
• Vascular changes;
1) Increased diameter, blood flow hence heat&
redness.
2) Endothelial cells express cell adhesion
molecules.
3) Increased vascular permeability hence
swelling & pain
4) Clotting in micro-vessels.

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