6

1. INNATE IMMUNITY
LECTURE 3
DR SADA MOEEZ

2. The Logic of Immunology
• •The immune system consists of a collection of cells, chemicals, processes, and mechanisms that function to
protect the body from foreign antigens, such as microbes (organisms such as bacteria, fungi, and parasites),
viruses, cancer cells, and toxins.
• •Beyond, the structural and chemical barriers which protect us from infection, the immune system consists of
two lines of defense; innate immunity and adaptive immunity.
• •The innate immunity represents the first line of defense against the antigen, and it is an antigen-independent
defense mechanism used by the host immediately or within hours of encountering an antigen.
• •On the other hand, adaptive immunity is antigen-dependent and antigen-specific and, therefore, includes a
lag time between exposure to the antigen and maximal response.
• •Immunology is focused on certain organs of the body like the bone marrow and the lymphatic system and the
white blood cells found in the blood.
• •These cells or organs producing these cells are responsible, directly or indirectly, for the defense mechanism
of the body against a pathogenic agent or other antigens.
• •These cells keep on circulating throughout the body via blood or lymph so that they can detect antigens
entering the body from different sources.

6. Body Lines of Defense- Innate and Acquired
Immunity
• There are two types of body responses against invaders:
• •Innate (Natural/Non-specific response) and Acquired (Adaptive/Specific) responses.
• •Innate responses occur to same extent no matter how many times the pathogen is exposed while
acquired responses are improved with the repeated subsequent exposure to foreign particles.
• •Innate responses employ phagocytic cells such as neutrophils, monocytes and macrophages, and
Natural Killer (NK) cells. However, adaptive responses involve antigen-specific B and T cells or
antigen-presenting cells (APCs).
• •Innate responses respond immediately to the foreign attack while adaptive take a longer time to react.
• •Innate responses fight against all invaders, hence termed as non-specific and acquired responses fight
against specific types of invaders, specific.
• •Innate response comprises of the first and second line of defense and acquired response includes the
third line of defense.

9. INNATE IMMUNITY

10. INNATE IMMUNITY

11. INNATE IMMUNITY

12. First Line of Defense
Basically, there are three types of lines of defense:
1. First line of defense
2. Second line of defense
3. Third line of defense
• The first line of defense is also known as an outside defense system.
• Physical, chemical, and biological defenses form the first line of defense against the invasion
of pathogens and these do not function independently and can overlap based on their
functions.
• The first line of defense consists of three basic components
• i)Physical defenses
• ii)Chemical defenses
• iii)Biological defenses

13. Components of innate immunity
• Anatomical barriers
• Physical
• Chemical
• Biological
• Cellular components
• Phagocutic cells
• Denritic cells
• NK cells
• Soluble proteins
• Complement
• Antimicrobial substances

14. First Line of Defense
• Physical defenses: These include physical barriers and mechanical defenses
which block the entry point of pathogens such as intact skin and mucus.
• Skin has three layers: The epidermis, the dermis, and the hypodermis. The
topmost layer, the epidermis is packed with keratin along with dead skin cells.
These dead cells are frequently being shed and replaced. The keratin is highly
water-resistant and mechanically tough hence, resists microbial growth.
• Nasal hairs filter air contaminated with microbes, dust, and dirt while
microscopic cilia lining the respiratory tract, sweep mucus and trap particles
inhaled towards body openings where they can be removed from the body.
• Mucous membranes lining respiratory, urinary, and reproductive tracts, produce
mucus, a slimy substance that traps foreign particles and directs them out of the
body by mechanical actions such as shedding, coughing, peristalsis, and flushing
of bodily fluids (e.g. urination, tears)

15. First Line of Defense
• ii) Chemical defenses comprise chemicals and enzymes in body fluids, a variety of plasma protein
mediators, cytokines, antimicrobial peptides, inflammation- eliciting mediators that destroys
pathogens on the outer body surface, at body opening, and on inner body linings.
• Sweat, tears, mucus, and saliva contain enzymes that kill pathogens. Lysozyme, an enzyme
found in tears, perspiration, and saliva can break down the cell walls of bacteria and kill them.
Likewise, secretory IgA function in a similar manner by attacking peptidoglycans in the cell wall
of bacteria. Antimicrobial peptides (AMPs) include dermcidin, cathelicidin, defensins, histatins,
and bacteriocins. AMPs are produced in response to pathogens on the skin
• Cerumen or ear wax contains fatty acids thereby lowering the pH between 3 and 5, it protects the
auditory canal from foreign particles like microbes.
• Gastric juice (pH 2-3) is highly acidic in nature and those pathogens that enter the stomach
through the oral cavity or nasal tract are destroyed by it.
• Urine flow, acidic in pH, kill microbes and direct out of the urethra.
• Serum (unsaturated fatty acids) reduce water loss as well as inhibit microbial growth however, it is
found to have certain compounds that facilitate to provide nutrition for certain microbes.

16. First Line of Defense
• iii) Biological defenses are provided by living microorganisms that are
friendly and beneficial.
• •These are resident natural flora that resides on our skin, in our bowel, and
in other places such as the mouth, gut, reproductive part, etc.
• •These prevent pathogen adherence and their colonization by creating an
acidic environment with the help of fermentation of sugars to acids,
occupying available cellular binding sites and competing with them for
available nutrients.
• •Thus, resident normal microbiota also contributes to chemical defenses as
they produce bacteriocins that perform the antibacterial activity.

17. Second Line of defense/cellular defense
• Origin, Differentiation, and Characterization of Cells of the
Innate Immune System
• •When the first line of defense is broken, the second line of defense
within our body „kicks‟ in and gets activated i.e. when the pathogens
successfully win the battle against barriers from the first line of
defense, they are next encountered by the second line.
• These include the polymorphonuclear leukocytes (white blood cells
containing a segmented lobular nucleus and include eosinophils,
basophils, and neutrophils), monocytes, and macrophages, each of
which is derived from hematopoietic precursor cells.
• There is no immunological memory.

18. Second Line of Defense
• Polymorphonuclear Leukocytes. Polymorphonuclear (PMN)
leukocytes are a population of cells also referred to as granulocytes.
• These include the basophils, eosinophils, and neutrophils.
Granulocytes are short-lived phagocytic cells that contain the enzyme-
rich lysosomes, which can facilitate destruction of infectious
microorganisms.
• They also produce peroxide, superoxide radicals, and nitric oxide,
which are toxic to many microorganisms.
• Some lysosomes also contain bactericidal proteins, such as lactoferrin.
PMN leukocytes play a major role in protection against infection.

19. Second Line of Defense
• Macrophages. Macrophages are phagocytes derived from blood monocytes .
• The monocyte itself is a small, spherical cell with few projections, abundant cytoplasm,
little e
• Following migration of monocytes from the blood to various tissues, they undergo further
differentiation into a variety of histologic forms, all of which play a role in phagocytosis,
including the following:
• • Kupffer cells, in the liver; large cells with many cytoplasmic projections
• • Alveolar macrophages, in the lung
• • Splenic macrophages, in the red pulp
• • Peritoneal macrophages, free-floating in peritoneal
• fluid
• Microglial cells, in the central nervous tissue Endoplasmic reticulum, and many
granules.

20. Second Line of Defense
• Dendritic Cells.These are differentiated from monocytes present in the
tissues located in contact with the external environment such as skin,
nose, lungs, etc.
• These cells link innate immunity and adaptive immunity. Once they
find foreign particles, they migrate to lymph nodes where they present
the antigen and interact with T cells and B cells that initiate an
adaptive immune response.
• These are named dendritic in the sense that they have their cellular
morphology in the form of a “tree-like” structure.

21. Second Line of Defense
• They are found as migrating dendritic cells in the blood.
• Non-migratory follicular dendritic cells (fDCs)
• Important for B cell responses
• Abundant in secondary lymphoid organs i.e spleen, lymph nodes
• Another type of dendritic cell is the plasmacystoid DC (pDC).
• Rare type of immune cells
• Known to secrete large quantity of TYPE 1 IF in response to viral infection.

22. Second Line of Defense
• Natural Killer (NK) cells: These are the only cytotoxic lymphocytes
belonging to the innate immune system and respond rapidly to virus-
infected cells or tumor cells without any priming or prior activation.
• Thus, these are best known for their “natural” killing ability
subsequently, help to detect and control early signs of cancer.
• Altered features of the membranes of abnormal cells

23. Unlike cytotoxic T lymphocytes, which recognize
target cells in an antigen-specific fashion due to their
expression of T cell receptors (TCRs), NK cells lack
antigen-specific receptors. How, then, do they seek and
destroy their targets? They do this by using a mechanism
involving cell–cell contact, which allows them to determine
whether a potential target cell has lost a particular selfantigen,
namely, major histocompatibility complex (MHC)
class I. MHC class I is expressed on virtually all nucleated
cells. NK cells express receptors called killer-cell inhibitory
receptors (KIR), which bind to MHC class I molecules
expressed on normal cells. When ligated, KIRs inhibit
downstream events that would otherwise cause the NK cell
to be activated, causing degranulation and destruction of
the target cells. Virus-infected or transformed (tumor) cells
have significantly reduced numbers of MHC class I mol�
ecules on their surfaces. Thus, when such cells encounter
NK cells, they fail to effectively engage KIRs and therefore
become susceptible to NK cell-mediated cytotoxicity

24. Second Line of Defense
• Natural Killer T Cells.
• Types of T cells that also have certain features of NK cells
• Present in thymus, spleen, liver
• Natural killer T (NKT) cells differentiate from thymic precursors
through signals originates from cortical thymocytes
• Like other T cells, these cells express TCRs, although with restricted
variability.
• Following activation, they secrete several regulatory cytokines,
including interleukin-4 (IL-4) and interferon-γ and kill target cells via
Fas–Fas ligand interactions that cause apoptosis.

25. Second Line of Defense
• Innate Lymphoid Cells: Innate lymphoid cells (ILCs) growing family of
immune cells that are same in phenotype and function to T cells but lack
TCR but can release an arrat of effector cytokines that match to T helper
cells.
• Found in lungs, skin, liver and gut.
• Functions:
• at the acute phase of infections, tissue remodeling, wound healing, and in
maintaining the epithelial integrity at mucosal sites.
• ILCs, which include a population called lymphoid tissue-inducer (LTi)
cells, share a lymphoid morphology, are dependent on the interleukin-2
receptor γ (IL-2Rγ) chain for their development but lack the rearranged
antigen receptors or markers that are expressed on cells of myeloid origin or
cells of the adaptive immune system

27. Second Line of Defense
• Basophils: These account for merely 1% of WBCs. These are known for
their inflammatory response associated with asthma and allergies as when
they are stimulated, they release histamine.
• Eosinophils are specialized white blood cells that capture parasitic infection
and boost inflammation. If you have too many, it's called eosinophilia.
• Neutrophils: These are the first immune cells to respond to intruders and
form approximately 65% of WBCs. They squeeze out through capillaries
into the site of infection as well as send out signals to alert other immune
cells. Thus, these are also known as „patrol tissue‟. Once they are released
from bone marrow, they have a short life span of about only eight hours.
• Mast cells: These exhibit functional similarities with basophils although
these reside within tissues. They show inflammatory response as a result of
activation with different inflammatory mediators. It releases inflammatory
histamine once it recognizes pathogens.

28. Complement System
• The complement system, also known as complement cascade.
• It consists of a series of serum proteins
• The complements exist as soluble inactive precursors which once
activated, a complement component may then act as an enzyme.
• Enzymatic chain reactions of this type are known as cascade reactions
and usually require a “trigger” to initiate the reaction chain.

29. Functions
• Complement triggers the following immune functions:
1. Membrane attack– by rupturing the cell wall of bacteria. (Classical
Complement Pathway)
2. Phagocytosis – by opsonizing antigens. C3b has most important
opsonizing activity. (Alternative Complement Pathway)
3. Inflammation – by attracting macrophages and neutrophils . (Lectin
pathway)

30. COMPLEMENT
• Complement proteins circulate in the blood in an inactive form.
• Each component takes its turn in a precise chain of steps known as the
complement cascade
• The end products are molecular cylinders called the membrane
attack complex (MAC), which are inserted into the cell walls that
surround the invading bacteria.
• This results in the development of puncture holes causing fluids to
flow in and out of the bacteria. Consequently, the bacterial cell walls
swell, burst (lysis), and the bacteria are killed.

32. Components of Complement

33. Classical Activation Pathway
• The classical pathway is triggered by activation of the C1-complex.
• The C1-complex is composed of 1 molecule of C1q, 2 molecules of
C1r and 2 molecules of C1s.
• When the first protein in the complement series (C1q) is activated by
an antibody (IgM/IgG) that has been made in response to a microbe
(e.g., bacteria)
• This initiates the chain of events resulting in the generation of MAC
that causes lysis of the microbe.
• is part of complement’s participation in the adaptive immune response

34. Alternative Activation Pathway
• It involves direct binding of certain complement components, such as
C3, to the surfaces of pathogens without the participation of antibody.
• This binding triggers a conformational change in the protein that
initiates the downstream cascade
• Leading to the Generation of MAC
• Followed by lysis of the microbe.
• is continuously activated at a low level
• Spontaneous C3 hydrolysis due to the breakdown of the internal
thioester bond

35. Lectin Activation Pathway
• The lectin pathway is homologous to the classical pathway, but with
the opsonin, mannose-binding lectin (MBL), instead of C1q.
• This pathway is activated by binding of MBL to mannose residues on
the pathogen surface
• Which activates the MBL-associated serine proteases, MASP-1, and
MASP-2 (very similar to C1r and C1s, respectively).
• Which can then split C4 into C4a and C4 band C2 into C2a and C2b.
• C4b and C2b then bind together to form the classical C3-convertase,
as in the classical pathway.

36. Opsonization
• Opsonization: refers to the process of coating of bacteria by the
opsonins.
• Opsonins: an antibody or other substance which binds to foreign
microorganisms or cells making them more susceptible to
phagocytosis.

41. Intracellular and Extracellular Killing of
Microorganisms
• Another hallmark of innate immunity is the ability of certain innate
immune cells to sample their environment by engulfing macroparticles
(endocytosis) or individual cells (phagocytosis).
• Endocytosis: the process by which a cell moves large amounts of
material, or non-dissolved particles into its cytoplasm from the outside
environment.
• This can occur either by pinocytosis, which involves nonspecific
membrane invagination, or by receptor-mediated endocytosis, a
process involving the selective binding of macromolecules to specific
membrane receptors.

42. Endocytosis
• Ingestion of the foreign macromolecules generates endocytic vesicles
filled with the foreign material, which then fuse with acidic
compartments called endosomes.
• Edosomes then fuse with lysosomes containing degradative enzymes
(e.g., nucleases, lipases, proteases) to reduce the ingested
macromolecules to small breakdown products, including nucleotides,
sugars, and peptides

43. Endocytosis
• Endocytosis is the case when a molecule causes the cell membrane to
bulge inward forming a vesicle
• The three types of endocytosis are:
• Pinocytosis (cellular drinking)
• Phagocytosis (cellular eating)
• Receptor mediated endocytosis

44. Pinocytosis (cellular drinking)
• Steps of the Process
• The basic steps of pinocytosis are described below:
• Step 1 – Initiation: In the initial stage, an inducer molecule, such as sugar, protein, or ion, comes in contact
with the plasma membrane. As a result, an ionic interaction occurs between the positively charged inducer
and the negatively charged cell membrane. This phenomenon triggers the binding of the molecule to the
membrane.
• Step 2 – Folding of the Membrane: Following binding, the cell membrane gets stimulated to fold inwards and
form a small open-ended pocket, or invagination, around the fluid containing the molecule.
• Step 3 – Invagination and Engulfment of the Fluid: The cell membrane continues to fold inwards with the
fluid and dissolved solutes in small pockets. Later, they start to reconnect at the open end of the invagination
to enclose the fluid and solutes.
• Step 4 – Detachment of the Pocket: This is the final stage when both ends of the invagination meet. It gives
rise to a vesicular structure called a pinosome that contains extracellular fluid and dissolved solutes.
Afterward, this pouch gets detached or pinches off from the cell membrane. The molecules present inside the
vesicle are eventually released to be used by other parts of the cell, thus completing the process.

46. Phagocytosis
• is the process of engulfment and destruction of solid particles such as bacteria,
dead tissues and foreign particles by the cells.
• Cells performing phagocytosis are called phagocytes
• Cells types are neutophils, monocytes and macrophages
• Many microorganisms release substances that attract phagocytic cells
• Phagocytosis may be enhanced by a variety of factors that make the foreign
particle an easier target. These factors, collectively referred to as opsonins
(the Greek word meaning “prepare food for”), consist of antibodies and
various serum components of complement.
• After ingestion, the foreign particle is entrapped in a phagocytic vacuole
(phagosome), which fuses with lysosomes, forming the phagolysosome. The
latter release their powerful enzymes, which digest the particle.

47. Phagocytosis
• Phagocytes can also damage invading pathogens through the
generation of toxic products in a process known as the respiratory
burst.
• Production of these toxic metabolites is induced during phagocytosis
of pathogens such as bacteria and catalyzed by a set of interrelated
enzyme pathways.
• The most important of these are nitric oxide (inducible nitric oxidase
synthase), hydrogen peroxide and superoxide anion (phagocyte
NADPH oxidase), and hypochlorous acid (myeloperoxidase), each of
which is toxic to bacteria.

50. Receptor mediated endocytosis
• Receptor-mediated endocytosis (RME), also called clathrin-
mediated endocytosis.
• Is a process by which cells absorb metabolites, hormones, proteins –
and in some cases viruses – by the inward budding of the plasma
membrane (invagination).
• This process forms vesicles containing the absorbed substances and is
strictly mediated by receptors on the surface of the cell.
• Only the receptor-specific substances can enter the cell through this
process.

53. INFLAMMATION
• The word “inflammation” comes from the Latin inflammare (to set on
fire).
• Function of phagocytic cells is their participation in inflammatory
reactions.
• As a physiologic process, inflammation
• is typically initiated by tissue damage caused by endogenous factors
(such as tissue necrosis or bone fracture) and by exogenous factors.
The latter includes various types of damage, such as mechanical injury
(e.g., cuts), physical injury (e.g., burns), chemical injury (e.g.,
exposure to corrosive chemicals), immunologic injury (e.g.,
hypersensitivity reactions

54. Causes of Inflammation
• Infective agents like bacteria, viruses and their toxins, fungi.
• Immunological agents like cell-mediated and antigen antibody
reactions.
• Physical agents like heat, cold, radiation.
• Chemical agents like organic and inorganic poison
• Inert materials foreign bodies

55. INFLAMMATION
• Most of the cells involved in inflammatory responses are phagocytic
cells, consisting mainly of the polymorphonuclear leukocytes that
accumulate within 30 to 60 minutes, phagocytize the intruder or
damaged tissue, and release their lysosomal enzymes in an attempt to
destroy the intruder.
• If the cause of the inflammatory response persists beyond this point,
within 4 to 6 hours, the area harboring the invading microorganism or
foreign substance will be infiltrated by macrophages and lymphocytes.
• The macrophages supplement the phagocytic activity of the
Polymorphonuclear cells, thus adding to the defense of the area.

56. Within minutes after injury, the inflammatory
process begins with activation of innate
immune cells responding to microbes
expressing PAMPs. As discussed earlier.
activation is stimulated by ligation of PRRs
and results in the release of proinflammatory
cytokines such as IL-1, IL-6, and tumor
necrosis factor-α (TNF-α)
These cytokines travel through the blood and
stimulate hepatocytes in the liver to secrete
acute phase proteins

57. Acute phase proteins are
defined as those proteins
whose serum concentrations
increase or decrease by at least
25 percent during
inflammatory states .
These are of 2 types
1. Positive APP
2. Negative APP

61. FEVER
• Induced by pyrogens
• Pyrogen is a polypeptide or polysaccharide which induces fever when released
into circulation.
• 2 types of pyrogens
• Exogenous pyrogens are molecules found outside of the body, such as
endotoxins from gram-negative bacteria.
• Some bacteria produce pyrogens that are known as endotoxins and exotoxins.
• Endotoxins are found in the cell wall of Gram-negative bacteria (LPS)
• Exotoxins are molecules that some bacteria make internally and secrete to the
outside.

62. FEVER
• Exposure of innate immune cells (monocytes and macrophages) to
LPS causes their release of cytokines called endogenous pyrogens
• IL-1
• TNF ALPHA
• IL-6

63. FEVER
• For example, the keratinocytes present in skin contain IL-1.
• when the skin is overexposed to the ultraviolet rays of the sun
(sunburn), keratinocytes are physically damaged, causing them to
release their contents, including IL-1.
• Within a few hours, IL-1 induces the hypothalamus to raise body
temperature (fever)—a phenomenon many have experienced after a
summer day at the beach, with accompanying chills and malaise.

64. Second Line of Defense
• Mechanism of the second line of defense
• With the invasion of the pathogen at the entry site, neutrophils come into action to engulf and destroy them.
• If they evade the action of neutrophils, then macrophages and dendritic cells go into battle which helps to phagocytose and
present antigens to T cells.
• Different internal defense includes phagocytosis, NK cells, inflammatory response, fever, and the complement system in
the second line of defense.
• Different cytokines are released by immune cells as well as infected cells which amplify the quantity of cytokine secretion.
• This triggers an inflammatory response, capillary dilation, and increased permeability of the capillary wall.
• Macrophages are involved in the resolution of the inflamed site and clearing up cellular debris. Cytokines also increase the
core body temperature resulting in fever.
• This increased temperature inhibits microbial growth and speeds up recovery and repair processes.
• NK cells detect viral infested cells or cancerous cells thereby inducing apoptosis. Similarly, different complement proteins
found in the blood serum, are attracted to pathogens tagged with the adaptive immune system.
• A cascade of binding of complement proteins to pathogens coats the pathogen which serves as a marker to detect the
presence of a pathogen in phagocytes and their digestion by the phagocytosis process.