The Lectin Pathway Originates with Host Proteins Binding Microbial Surfaces
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The Lectin Pathway Originates with Host Proteins Binding Microbial Surfaces Lectin: proteins that bind to a carbohydrate MBL (mannose-binding lectin):

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The Lectin Pathway Originates with Host Proteins Binding Microbial Surfaces

Lectin: proteins that bind to a carbohydrate

MBL (mannose-binding lectin):

- a protein which binds to mannose residues on glycoproteins or carbohydrates on the surface of microorganisms (structurally similar to C1q)

MASP-1 & MASP-II: MBL-associated serine protease

(structurally similar to C1r and C1s)

  • MBL is induced during Microbial Surfacesinflammatory responses.

  • After MBL binds to the surface of a microbe,

  • MBL-associated serine proteases, ) MASP-1 and

  • MASP-2,( bind to MBL.

  • The MBL-MASP-1&II complex mimics the

  • activity of C1r and C1s, and causes cleavage and

  • activation of C4 and C2.

  • Thus, the lectin pathway is Ab-independent.

  • It is an important innate defense mechanism comparable

  • to the alternative pathway, but utilizing the elements

  • of the classical pathway, except for the C1 proteins.

Q2: Why doesn’t mannose-binding lectin (MBL) Microbial Surfaces

bind to host carbohydrates?

A2: Mammalian cells normally have sialic

acid residues covering the sugar groups

recognized by MBL and are not a target

for binding.

Biological Effects Mediated by Complement Microbial Surfaces

1. Cell lysis

The membrane-attack complex can lyse a broad spectrum of cells:

G(-) bacteria




nucleated cells (tumor cells)

Because the activation of alternative and lectin pathways is Ab-independent, these pathways serve as important innate

immune defenses against infectious microorganisms.

2. Inflammatory response Microbial Surfaces

- Various peptides generated during activation of

complement play a role in the development

of an effective inflammatory response.

- C3a, C4a, C5a (called anaphylatoxin) bind to

complement receptors on mast cells and basophils

and induce degranulation with release of histamine

and other mediators.

- The anaphylatoxins also increased vascular permeability,

extravasation, and chemoattraction (induced by

C5a, C3a, and C5b67)

3. Opsonization Microbial Surfaces

- C3b is the major opsonin of the complement system, although C4b also have opsonizing activity.

binds to the surface of microbes

4. Viral neutralization Microbial Surfaces

- Formation of larger viral aggregates reduces the

net number of infectious viral particles.

- The deposits of Ab and complement on viral

particle neutralizes viral infectivity by blocking

attachment to susceptible host cells and facilitates binding of the viral particle.

5. Clearance of Microbial Surfaces

immune complexes

Innate immunity complement
Innate Immunity: Complement Microbial Surfaces

Comparison of 3 pathways
Comparison of 3 Pathways Microbial Surfaces

Inflammation: Microbial Surfaces

Inflammation is non specific response to tissue damage resulting from a variety causes, including heat, chemicals, ultraviolet light, cuts and pathogens.

  • There are two types:

  • Acute inflammation: develops quickly and is short lived, is typically beneficial and result in the elimination of whatever condition precipitated.

  • Chronic inflammation: develops slowly, lasts a long time, and can cause damage to tissue, resulting in disease .

Acute inflammation is an important part of the second line of defense because it results in:

  • dilatation and increases permeability of blood vessels.

  • migration of phagocytes ( diapedesis).

  • tissue repair

Components of Innate of defense because it results in:Immunity Inflammation

  • Non-specific, physiologic reaction of vascular tissue to injury and microbial invasion.

  • The inflammatory process can be divided into two sequential steps:

    • Vascular Phase - dilation and increased permeability of blood vessels with the accumulation of fluid and cells at the site of injury.

    • Cellular Phase - the activation of specialized blood and connective tissue cells that destroy or wall off injurious agents and clear debris so that tissue repair can take place.

    • Cells important to inflammation include neutrophils, basophils, eosinophils, lymphocytes, monocytes, and platelets.

Signs of acute inflammation
SIGNS OF ACUTE INFLAMMATION of defense because it results in:

Heat Redness Swelling Pain Loss of function

Cytokines of defense because it results in:

  • An important group of small molecular weight polypeptides that are produced by lymphocytes, macrophages, and other connective tissue cells.

  • The function of cytokines is to regulate growth and differentiation of blood, lymphoid, and connective tissue cells.

  • They help to orchestrate many aspects of innate and acquired immunity as well as wound healing, and the production of blood cells.

  • Inflammatory cytokines: of defense because it results in: Interleukin-1 (IL-1) and Tumor Necrosis Factor (TNF)

  • induce the fever, fatigue, and decreased appetite.

    • Activate leukocytes and other inflammatory cells,

    • increase vascular permeability.

    • Activate tissue-degrading enzymes.

    • Activate the connective tissue cell involved in tissue repair.

Stages of inflammation 1 vascular phase
Stages of Inflammation: of defense because it results in:1. Vascular Phase

  • Vasoconstriction - Immediately following injury, tightening of blood vessels

  • Complement activation and mediator release –

    • Activated complement opsonize microbes/injured tissue and release of histamine and serotonin from nearby mast cells

    • Formation of bradykinin and initiation of synthesis of prostaglandins and leukotrienes

  • Vasodilation –

    • Histamine, and other vasodilators, cause relaxation of smooth muscle in arteriolar and capillaries walls.

    • Vessels dilate providing increased blood flow to the injured tissue.

    • Causes redness/heat

  • Increased vascular permeability of defense because it results in: –

    • endothelial cells lining blood vessels contract slightly creating gaps between the cells that allow plasma to escape into surrounding tissues.

    • Plasma delivers antibodies and other antimicrobial substances to the site of injury.

    • Fibrinogen from plasma also clots and serves as a temporary barrier to bacterial invasion.

Stages of inflammation 2 cellular phase
Stages of Inflammation: of defense because it results in:2. Cellular Phase

  • Adhesion – white blood cells (WBC’s) stick to inner surface of blood vessel walls

  • Diapedesis WBC's then to squeeze between the contracted endothelial cells and migrate in an ameba-like fashion into the extravascular space

  • Chemotaxis - Once in tissue, the WBC's begin to migrate towards the site of injury or microbial invasion. WBC's are attracted by chemotactic agents that have been released at the site of injury. WBC's apparently have surface receptors for chemotactic agents which cause them to move in the direction of increasing concentrations of the chemotactic substance.

Phagocytosis of defense because it results in: –

When WBC's arrive at the site of tissue injury or microbial invasion they become very active and begin engulfing bacteria (or other foreign) that have been opsonized by complement or antibodies; through various mechanisms, bacteria and other foreign substances are destroyed and degraded after phagocytosis

Increase permeability during inflammation of defense because it results in:

Mechanisms of inflammation
MECHANISMS OF INFLAMMATION of defense because it results in:

Emigration of Neutrophils of defense because it results in:

Inflammation outcome resolution

Chronic Inflammation of defense because it results in:









Inflammation Outcome - Resolution

Fever: of defense because it results in:

is a body temperature more than 37 ºC.

- Fever results due the presence of chemicals called pyrogens.

- Pyrogens trigger the hypothalamic thermostat to reset at hightemperature.

Pyrogens include of defense because it results in::

- bacterial toxins

- cytoplasmic contents of bacteria that are released upon lysis Ab-Ag complexes.

- interleukin-1 a pyrogen released by phagocytes that have phogocytized bacteria

Steps of fever of defense because it results in:

Determinants of innate immunity of defense because it results in:

Innate immunity is genetically controlled and varies widely with species, race and to less extent between individuals.

  • Species and race:

  • -Man and guinea pigs are highly susceptible to diphtheria while rats are not.

  • -Man is susceptible to common cold while dogs are not.

  • -American Indian and Negro are more susceptible to tuberculosis than white races.

B) Individual differences: of defense because it results in:

Age:the very young and aged are particular liable to infection.

Nutrition: Malnutrition and starvation predispose to infection by decreasing the total white cell count and phagocytosis.

Hormones: Some endocrine diseases cause a decrease in resistance to infection such as diabetes mellitus, hypothroidism and adrenal dysfunction.