4.1 Communicable diseases, disease prevention and the immune system

What are T lymphocytes?

A type of white blood cell (lymphocytes) that recognise non-self antigens on the infected cell's membrane, and destroy them by inducing apoptosis

Where do T lymphocytes mature?

Thymus gland

Types of T-lymphocytes:

- T helper cells

- T killer cells

- T memory cells

- T regulator cells

What type of response do T-lymphocytes induce?

Cell Mediated response

Cell Mediated Response

1. T helper cells have specific complementary receptors that bind to the antigens on APCs via clonal selection

2. They then undergo clonal expansion via mitosis

3. They form clones of identical activated T-helper cells that all carry the correct antigen to bind to a particular pathogen.

4. Cloned T cells may also produce T-killer cells, T-memory cells, and T-regulatory cells

T killer cells

They are specific for the presented antigen and destroy the pathogen carrying the antigens and infected cells

T memory cells

• They are part of the immunological memory

• They live for ages and give a rapid response if the pathogen invades the body again

What happens if a T memory cell meets an antigen a second time?

If a T memory cell meets an antigen for a second time, they divide rapidly to form a huge number of clones of T killer cells, that destroy the pathogen.

What do T regulator cells do?

They suppress the immune system, acting to control and regulate it

Why is the T regulator cell important?

They stop the immune response once a pathogen has been eliminated, and ensure the body recognises self antigens and does not set up an autoimmune response.

What is humoral immunity?

This is when antibodies attack antigens that are free in the plasma.

What does the humoral immune system produce?

Antibodies that are soluble in the blood and tissue fluid and are not attached to cells

What type of lymphocyte are in the humoral immune system?

B lymphocytes

B lymphocytes have what?

antibodies on their cell surface membrane (IgM)

Humoral Immunity Process

1. When a pathogen enters the body, it will have specific antigens.

2. A B cell with complementary antibodies will bind to the antigens on the pathogen.

3. This B cell then engulfs the pathogen and processes the antigens to become an APC

4. Activated T helper cells bind to the B cell APC. This is known as clonal selection.

5. Interleukins produced by the activated T helper cells activate the B cells

6. The activated B cell undergoes clonal expansion via mitosis to give plasma cell clones and B memory cell clones.

7. Plasma cell clones produce antibodies that are complementary to the antigens, bind to them, and disable them.

   Or they act as opsonins or agglutinins.

What are opsonins?

Opsonins are molecules that bind to antigens on the surface of pathogens, making them easier for phagocytes to recognize and engulf, thereby increasing the rate of phagocytosis

What are agglutinins?

Antibodies that cause the red blood cells to clump together.

Why do we get ill?

The primary immune response take a while to become fully effective against a particular pathogen.

This is why we get ill, as the symptoms are the result of the way our body reacts when the pathogens are dividing before the primary immune system can fight back

Cloned B cells develop into B memory cells. What happens if the body is infected by the same pathogen again?

The B memory cells divide rapidly to form plasma cell clones.

What do the plasma cell clones do?

They produce the correct antibody, and wipe out the pathogen very quicky, before it can cause the symptoms of disease.

What is clonal selection?

The process of matching the antigens on APCs with complementary receptors on B and T lymphocytes.

What is clonal expansion?

The process where an activated B or T lymphocyte divides by mitosis to produce many clones.

• Activated B cells divide into plasma cells and B memory cells.

• Activated T helper cells divide into more T helper cells, and T killer cells, T memory cells, T regulatory cells.

What is an immune response?

Response to an antigen and involves lymphocytes

What is cell mediated immunity?

An immune response that involves T Cells

Where do T cells mature?

Thymus gland

What is humoral immunity?

An immune response involving B cells

Where do B cells mature?

Bone marrow

Specific Immune Response Process

1. In the non-specific defence system, macrophages engulf and digest pathogens in phagocytosis.

2. They then process the antigens from the surface of the pathogen to form antigen presenting cells (APCs)

3. The T helper cells have specific and complementary receptor to the antigens on the APC.

   This is clonal selection.

4. These T helper cells will undergo clonal expansion by mitosis to produce:

   • More T helper cells

   • T Killer cells

   • T memory cells

   • T regulatory cells

5. T helper cells also release interleukins and cytokines

6. Interleukins and cytokines trigger B cells to undergo clonal expansion by mitosis

7. When the B cells undergo clonal expansion, they will produce:

   • B memory cells

   • Plasma cells

What do T memory cells do?

T memory cells give a rapid response if this pathogen invades again

What do T killer cells do?

T killer cells that are specific for the presented antigen will then then destroy infected cells

How do non-specific defences work?

The body has a number of barriers to prevent the entry of pathogens.

Name the primary non-specific defences against pathogens in animals

• Skin

• Blood clotting

• Wound repair

• Inflammation

• Expulsive reflexes

• Mucous membranes

Skin

• The outer layer of cells is the epidermis. It acts as a barrier.

• Sebum is an oily substance that prevents the growth of pathogens

• Evaporation of sweat from the skin leaves behind a salt residue

• Skin flora (healthy microorganisms) try to outcompete any pathogens

Blood Clotting & Wound Repair

1. Tissue becomes damaged

2. Platelets are released

3. These platelets release substances via a cascade of events, resulting in the formation of fibrin

4. Fibrin forms a mesh of fibres in which platelets and blood cells get trapped.

5. This dries to form a scab, which prevents bleeding

Inflammation

A condition in which the body reacts to injury, irritation, or infection.

Is inflammation a local response?

Yes

In inflammation, what happens when a tissue is damaged?

Mast cells detect an infection, and they release histamines and cytokines

What do cytokines do?

They attract more phagocytes to the infected area so they can dispose of the pathogens by phagocytosis

What does histamine do?

Arterioles dilate, so there is increased blood flow to the area

This causes localised heat and redness

The increase in temperature helps prevent the pathogens from reproducing

What does histamine do to capillaries?

The capillaries become more leaky, allowing for more tissue fluid to form.

This brings more white blood cells to the area, an also causes more fluid to enter into the lymphatic system.

The Lymphatic System

- Excess tissue fluid is drained into the lymphatic system

- Pathogens will enter the lymph and be transported to the lymph nodes

- Lymph nodes swell to produce phagocytes and lymphocytes

Mucous Membranes

Airways and the digestive system are exposed and lined with mucous secreting cells (goblet cells).

How do mucous membranes prevent pathogens from entering?

• Mucus in the airways (trachea, bronchi and bronchioles) trap the particles (virus, bacteria, pollen etc) that we breathed in

• The particles are then moved towards the back of the throat by cilia

• Cilia are small hair-like structures on the surface of cells that beat and move in a coordinated manner to shift material along the epithelial cell

Expulsive Reflex

Cough or sneeze

How do expulsive reflexes prevent pathogens from entering?

- When a pathogen irritates the lining of an airway it can trigger an expulsive reflex

- This expelled air contains secretions from the respiratory tract along with the foreign particles that have entered

Other Primary Defences

- Stomach acid

- Antibodies in tears

- Wax in ears and nose

- Acidic conditions in uterus

Lysozymes

Antimicrobial enzymes that breakdown the cell wall of bacteria.

Where are lysozymes found?

Tears

Medicines

Drugs that are used to treat or prevent diseases or other conditions

Penicillin

• From a mould

• Antibiotic

Docetaxel

• From yew trees

• Cancer treatment

Aspirin

• From willow bark

• Painkiller

Digoxin

• From foxgloves

• Powerful heart drug used to treat atrial fibrillation and heart failure

Why is biodiversity loss bad for medicines?

Biodiversity contains animals, plants, and microorganisms which could have medicinal properties important for a live saving drug

Why do scientists design drugs using computer programmes?

• Models of potential drug molecules can be built up

• These models are targeted at particular areas of the pathogen.

• Computers can also search through chemicals to isolate any with a useful action against a specific group or feature of a pathogen (or against the mutated cells in a cancer)

What is pharmacogenetics?

The use of genome information in the choice of drugs

What is synthetic biology?

The use of techniques like genetic engineering to medicinal benefit. Bacteria can be genetically modified to produce drugs such as insulin.

What is nanotechnology?

A strand of synthetic biology where tiny synthetic particles are used for biological purposes. _{Like delivering drugs to very specific sites within the cells of pathogens or tumours.}

Primary & secondary immune response

1. Initial exposure

2. Primary immune response produces a small concentration of antibodies

3. Secondary exposure

4. Secondary immune response produces a high concentration of antibodies

What are the 2 reasons why there is a time delay between initial exposure and the primary immune response?

• The clonal selection and expansion of specific T cells and B cells

• The synthesis of antibodies

What do B cells do in the primary immune response?

Some B cells differentiate during clonal expansion to become plasma cells and memory cells

What is the secondary immune response?

The response to the secondary exposure to the pathogen or antigen or disease

Explain the graph

• During the primary response, there are no memory cells.

• After the immune response, there are now T and B memory cells that have been produced during cell mediated and humoral response

• If the same foreign antigen re-enters, B memory cells recognise the antigen.

• B memory cells divide quickly and differentiate into plasma cells to produce antibodies and more memory cells.

Is the secondary immune response quick?

Yes

Why is the secondary immune response quicker?

Because memory B-cells and Memory T-cells can recognise the antigen and produce the correct antibodies quicker.

Mention these key terms in an answer:

Clonal selection

Clonal expansion

Plasma cells & antibody concentration

How do T memory cells play a part in the secondary immune response?

T-lymphocytes differentiate into memory cells, producing:

• Memory T helper cells

• Memory T killer cells

What will T memory cells do?

Like B memory cells, they remain in the body for a long time and provide long term immunity

What are phagocytes?

Specialised white blood cells that engulf and destroy pathogens

What are the 2 types of phagocytes?

Neutrophils and macrophages

Difference between neutrophils and macrophages

• Neutrophils have multi-lobed nuclei, which makes it easier for them to squeeze through small gaps to get to the site of infections. It also has granular cytoplasm, containing many lysosomes that contain enzymes used to attack pathogens.

• Macrophages have simpler round nuclei

Phagocytosis Process

1. Pathogens produce chemicals that attract phagocytes

2. Pathogen is engulfed and phagosome is formed

3. Lysosome fuse with the phagosome to form a phagolysosome

4. Hydrolytic enzymes in the phagolysosome break down (hydrolyse) the pathogen

5. Harmless products like the amino acids are absorbed into the cytoplasm

6. The digested pathogen combines with MHC in the cytoplasm. The MHC-antigen complex is displayed on phagocyte membrane, making an antigen presenting cell (APC)

What are cytokines?

Released from T helper cells to trigger clonal expansion of B cells

What are opsonins?

Molecules that bind to antigens on the surface of pathogens, making them easier for phagocytes to recognize and engulf, thereby increasing the rate of phagocytosis

Neutrophil Structure

• Multi-lobed nuclei, which makes it easier for them to squeeze through small gaps to get to the site of infections.

• Granular cytoplasm, containing many lysosomes that contain enzymes used to attack pathogens.

Lymphocyte Structure

Spherical nucleus

Monocyte Structure

Unilobular nuclei (kidney shaped - different to neutrophil)

Erythrocyte Structure (red blood cell)

Biconcave disc

Autoimmune Disease Definition

An abnormal immune response where the body's immune system mistakenly attacks its own cells and tissues, causing inflammation and damage.

When do autoimmune diseases appear to happen?

They are unable to recognise self antigens and instead treats them as non-self/foreign

What is a treatment for autoimmune diseases?

Immunosuppressant drugs

Antinflammatories

What do immunosuppressant drugs do?

They prevent the immune system from working

Why are immunosuppressants not that good?

They deprive the body of its natural defences against communicable diseases

What body part does Rheumatoid arthritis affect?

Joints

What's the treatment of Rheumatoid arthritis?

- No cure

- Anti-inflammatory drugs

- Steroids

Black Sigatoka

Fungi

AIDS

Virus

Influenza

Virus

Malaria

Protoctista

Tuberculosis

Bacteria

Potato/tomato late blight

Protoctista

Ring rot

Bacteria

Athlete's foot

Fungi

What are antibodies?

Y-shaped glycoproteins called immunoglobulins which bind to a specific antigen on the pathogen/toxin that has triggered the immune response.

Describe the structure of an Antibody

• Protein

• Made of 4 polypeptide chains (2 light polypeptide chains, 2 heavy polypeptide chains)

• More than 1 variable regions complementary and specific to an antigen

• Hinge region allowing it to bind to more than one antigen (or pathogen)

• Constant region allows it to find phagocytes, aiding phagocytosis

What is an antigen?

A toxin or foreign substance that induces an immune response in the body, especially the production of antibodies.

Antibody Structure

Antibody Chains

Antibodies are made up of 2 identical long polypeptide chains called the heavy chains, and 2 shorter identical chains called the light chains.

How are the chains held together in the antibodies?

Disulphide bridges hold and join the polypeptides, holding them in shape.

Binding Sites

• Antibodies bind to antigens in a lock and key mechanism

• It is a different shape on each antibody and gives the antibody its specificity