U4 AOS 1 (O1) - How Do Organisms Respond to Pathogens?

U4 - How Does Life Change and Respond to Challenges?

What are the study design dotpoints for this AOS?

• Physical, chemical and microbiota barriers as preventative mechanisms of pathogenic infection in animals and plants

• Initiation of an immune response, including antigen presentation, the distinction between self-antigens and non-self antigens, cellular and non-cellular pathogens and allergens

• The innate immune response including the steps in an inflammatory response and the characteristics and roles of macrophages, neutrophils, dendritic cells, eosinophils, natural killer cells, mast cells, complement proteins and interferons

Antigens

• Molecules or parts of molecules that stimulate an immune response

  • Self-antigens are those that are not foreign and are thus usually tolerated by the immune system.

  • Non-self antigens can be identified as invaders and can be attacked by the immune system.

MHC Receptors

• The major histocompatibility complex (MHC) is a group of receptor proteins present on the surface of body cells.

• It helps the immune system distinguish the body's own proteins from proteins made by foreign invaders such as viruses and bacteria.

Pathogens

• A pathogen or infectious agent is a biological agent that causes disease or illness to its host. 

• Pathogens can be cellular or non-cellular.

Celllar pathogens

• Bacteria

  • Can be intracellular or extracellular

  • Some bacteria produce toxins

    • Exotoxin – secreted toxins

    • Endotoxin – parts of the membrane that are released when the bacteria dies

• Fungi

  • Opportunistic pathogens (eukaryotic)

• Protozoa

  • Single celled eukaryotes

• Parasites

• Disease causing organisms (e.g worms)

Non-cellular pathogens

• Viruses

  • Can only replicate within a host cell

  • Very small, much smaller than bacteria.

  • Contain DNA or RNA enclosed in a protein shell called a capsid

• Prions

  • Infectious particles made of protein.

  • Normal proteins fold into abnormal structures.

• Viroids

  • Simpler form of virus that lack a capsid

Allergens

• Any antigen that causes the immune system to produce an overreaction when exposed.

Divisions of immunity

• When the body recognises a non-self antigen an immune response begins

• Innate Immunity:

  • Non-Specific Immunity

• Adaptive Immunity:

  • Specific Immunity

Innate immunity

• Non-specific immune response to a non-self antigen.

• Uses the same response regardless of the pathogen (non-specific).

• Very fast.

Adaptive immunity

• Specific immune response to a non-self antigen.

• Uses specific immune cells targeted against a particular pathogen.

• Slower than innate.

Humoroal vs Cell-Mediated

Lines of Defence

1st (INNATE)

• Consists of physical, chemical and microbiological barriers to prevent pathogens from gaining entry to the body.

If breached THEN

2nd (INNATE)

• Actions of immune cells and soluble proteins mounting a rapid but non-specific attack against pathogens that gain entry to the body.

3rd (ADAPTIVE)

• Actions of immune cells and antibodies tailored specifically to attack each invading pathogen. 

1st Line

Types of barriers:

• Physical = PINK

• Chemical = BLUE

• Microbiological = PURPLE

Physical Barriers

• In animals

  • Intact skin

  • Mucous Membranes

• In plants

  • Cuticle

  • Thick Bark

  • Stomata

  • Cell Wall

  • Leaf Orientation

  • Thorns and Spikes

Chemical Barriers

• In animals

  • Enzymes

  • Sebum (Proteins)

  • Various Chemicals

• In plants:

  • Enzymes

  • Various chemicals

Microbiological Barriers

In animals only

• Normal flora (non-pathogenic bacteria)

Second Line of Defence - Pathogen has entered the tissues and/or bloodstream.

E.g. You fall and graze your knee, the intact skin has been broken and a bacteria has travelled into your surrounding tissues.

How will you know you have an infection?

• Redness

• Inflammation

• Pus

Second Line of Defence - Immune Cells:

White blood cells (leukocytes) are found in the blood and the lymph and they migrate to the site of infection

Histamine: Released by _ cells to attract white blood cells to the site of infection.

Other chemical messengers play a role in this recruitment as well.

mast

All the Immune Cells Involved in the Second Line of Defence

• Leukocytes

• Monocytes

• Neutrophils

• Esoinophils

• Basophils

• Natural Killer Cells

• Macrophages

• Dendritic Cells

• Mast Cells

Phagocytosis

• Phagocytosis involved the engulfment and destructions of a pathogen.

• Phagocytes/Phagocytic cells

  • Neutrophils

  • Macrophages (APC)

  • Dendritic cells (APC)

Steps in Phagocytosis

1) The pathogen is engulfed by the plasma membrane of the phagocyte.

2) The pathogen is engulfed in a vesicle called a phagosome.

3) Lysosomes fuse with the phagosome (forming a phagolysosome).

4) Toxic chemicals from the lysosome digest and destroy the pathogen.

5) Indigestible material is discharged from the phagocytic cell by a process of exocytosis.

Macrophage

• Mature monocytes important in innate and adaptive immunity.

• Large phagocytic cells found in most tissues.

Neutrophil

• Most abundant white blood cell

• Phagoctyic cells

Dendritic Cell

• Phagocytic cells that can present antigens on their surface.

Eosinophil

• Granulocyte present in respiratory, gastrointestinal and urinary tracts.

• Granules contain toxic chemicals and histamine.

• Help fight large parasites.

Mast Cell

• Mediate inflammatory responses.

• Contain cytokines, histamine and heparin.

• Play a role in an allergic response

Natural Killer Cell

• NK Cells destroy virus-infected cells through apoptosis, destroying the self cell and any virus it contains.

Natural Killer Cell - Processes

Degranulation:

1. Perforin molecules punch holes in the plasma membrane of target cells. This enables proteases into the target cell.

2. Proteases induce apoptosis once inside the target cell.

Symptoms of Inflammation

Redness — due to vasodilation of blood vessels

Pain

Heat — due to increased blood flow

Swelling — due to the movement of fluid into tissues after vasodilation

Pus — due to dead phagocytes and cell debris

Stages of Inflammation (stage/list)

1. Vascular Stage

2. Cellular Stage

3. Resolution Stage

Stages of Inflammation (expand/elaborate)

1. Vascular Stage

• Damaged cells release cytokines and prostaglandins

• These attract neutrophils

• Mast cells release histamine which dilates blood vessels and increase permeability of the blood vessels

2. Cellular Stage

• Immune cells migrate to the infection site

• These cells release more cytokines and histamine to recruit more immune cells to the infection site

• Neutrophils and macrophages phagocytose bacteria.

Complement Proteins

• Proteins that are dissolved in the plasma of blood. They add to or ‘complement’ the function of immune cells.

• Part of the innate immune system

• Can be activated by antibodies in the adaptive system

Complement Proteins - Mode of Action

Complement acts by:

• Opsonise pathogens

• Recruit immune cells in an inflammatory response (chemotaxis)

• Destroy bacteria through lysis or a membrane-attack complex (creates a pore in the plasma membrane allowing fluid to enter)

• Part of the innate immune system

• Can be activated by antibodies in the adaptive system

What are antibodies?

• proteins produced by plasma cells in response to antigens and which react specifically with the antigen that induced their formation

• also called immunoglobulins

What are immunoglobins?

• antigenbinding proteins produced by B cells and released in blood and lymph

What is inflammation?

• an innate reaction by the immune response to foreign particles or injury resulting in redness and swelling

What is cytotoxic T cells?

• T cells that are activated by cytokines to bind to antigen–MHC-I complexes on infected host cells and kill infected body cells

What are Self-antigens?

• Antigens on cells that are recognised by self￾receptors as being part of the same body

What are non-self antigens?

• antigens that do not belong to the body’s own cells

What are cell surface receptors?

• regions of a trans-membrane molecule exposed at the surface of a cell that act in cell signalling by receiving and binding to extracellular molecules

What is MHC?

• receptor proteins on the surface of cells that identify the cells as ‘self’

What is are autoimmune disease?

• diseases in which the immune system fails to identify ‘self’ material and makes antibodies against the body’s own tissues

What are human leukocyte antigens?

• antigens present on human cell surfaces that determine the ‘self’ status of a person’s cells

What is MHC-I?

• a type of major histocompatibility complex found on nucleated cells