Immunology

Question 1

What is a pathogen?

Answer 1

any organism that can produce disease

Question 2

What organisms can be classified as pathogens?

Answer 2

bacteria e.g. TB
protoctista e.g. malaria
viruses e.g. HIV
fungi e.g. athelete’s foot

Question 3

What is the difference between communicable and non-communicable diseases?

Answer 3

A non-communicable disease (NCD) is a disease that is not transmissible directly from one person to another e.g. Parkinson’s disease

A communicable disease is one that can be transmitted from one person to another e.g. a bacterial infection

Question 4

What are the modes of transmission for communicable diseases?

Answer 4

Transmission can be direct or indirect

Question 5

What is direct transmission?

Answer 5

Direct contact transmission occurs when there is physical contact between an infected person and a susceptible person

E.g. blood transfusions, sexual fluids, oral secretions etc…

Question 6

What is indirect transmission?

Answer 6

occurs when there is no direct human-to-human contact

infection can spread by a vector e.g. mosquitoes carrying malaria

or by aerosol droplets (sneezing or coughing)

Question 7

What barriers do plants have to stop pathogens?

Answer 7

Physical barriers: 
Wax
Cuticle
Cell wall
Stomata
Callose deposition

Chemical barriers:
Nutrient deprivation
pH
Phytoanticipins
Plant defensins

Question 8

How do plants respond to infection

Answer 8

1. Receptors in the cells respond to molecules from the pathogens, or to chemicals produced when the plant cell wall is attacked.
2. This stimulates the release of signalling molecules that appear to switch on genes in the nucleus.
   This in turn triggers cellular responses, which include:
   - Producing defensive chemicals
   - Sending alarm signals to unaffected cells to trigger their defences
   - Physically strengthening the cell walls

Question 9

What is callose deposition (plant immunology)?

Answer 9

When plants are attacked by pathogens, they produce high levels of a polysaccharide called callose

• Callose is synthesised and deposited between cell walls and cell membranes in the cells next to the infected cells - acting as barriers, preventing the pathogens entering the plant cells around the site of infection.
• Large amounts of Callose continue to be deposited in cell walls after initial infection. Lignin is added, making the mechanical barrier even thicker and stronger.
• Callose blocks sieve plates in the phloem, sealing off the infected part and preventing the infection from spreading.
• Callose is deposited in the plasmodesmata between infected cells and the neighbouring cells, sealing them off from healthy cells.

Question 10

What chemical defences are available to a plant?

Answer 10

Many plants are able to produce powerful chemicals that can either repel the insect vectors of disease or kill invading pathogens.

1) Insect repellents
2) Insecticides e.g. caffeine can kill some
3) Antibiotics
4) Lysosomes (like animal cells!)
5) Anti-fungals

Question 11

What non-specific defences against

pathogens are found in animals?

Answer 11

skin, blood clotting, stomach acid
wound repair, inflammation, expulsive reflexes and
mucous membranes (anything that produces mucus e.g. goblet cells in your airways - mucus traps bacteria)

Question 12

Give two examples of phagocytes

Answer 12

Neutrophils

Antigen presenting cells

Question 13

Summarise phagocytosis

Answer 13

Different ways of doing this, so don’t worry about exact wording here and if you want something to memorise use your textbook as I am writing this myself, but key points:

Phagocytosis is one of the main mechanisms of the innate immune defense (and so it is one of the earliest responses)

• Phagocytes have receptors on their cell surface membrane that allows them to bind to the pathogen.
• The phagocyte will then engulf the pathogen into itself (a form of endocytosis) to form a ‘phagosome’
• Lysosomes inside the phagocyte will then fuse with the phagosome (these have low pH and contain hydrolytic enzymes e.g. LYSOZYMES
• These lysozymes destroy the engulfed pathogens’ cell wall (hydrolysis)
• Other components of the pathogen are similarly broken down and the debris recycled

Question 14

What is the role of cytokines in phagocytosis?

Answer 14

Cells in the vicinity of the infection release cytokines which promote chemotaxis of phagocytosis

Question 15

What is the role of opsonins in phagocytosis?

Answer 15

Make it easier to phagocytose the pathogen by providing a binding target

Question 16

What are the 4 types of T cell?

Answer 16

Cytotoxic T killer cells (CD8+)
T memory cells
T helper cells (CD4+)
T-reg cells (CD4+)

Question 17

What cells produce antibodies?

Answer 17

B cells (plasma cells)

Question 18

Summarise clonal selection and expansion in relation to humoral immunity

Answer 18

Okay this can be worded in different ways and can be a bit weird because different sources discuss it differently. We need to first point out that “humoral” means relating to antibodies

In terms clonal selection and expansion it can involve APCs “showing the antigen” to the T cell and once the T cell with a complementary receptor (clonal selection) is found this undergoes clonal expansion (mitosis)

With B cells:

• The humoral response is best at fighting pathogens which are free in the bodily fluids
• Free antigen binds to a complementary B cell receptor, activating the B cell (clonal selection)
• The pathogen is endocytosed, and the antigen presented on the plasma membrane
• T helper cell binds to the presented antigen and stimulates the B cell to divide by mitosis (clonal expansion)
• The B cell differentiates to plasma and later memory cells