The Immune System: Pathogens, Infection and Defenses

Question 1

What is a pathogen?

Answer 1

An agent that potentially infects and damages host cells.

Question 2

What are examples of potential agents within infectious diseases, in order of size/complexity?

Answer 2

• Prions: Non-living, but have life-like properties
• Viruses
• Bacteria
• Protists
• Fungi: Eukaryotes, multicellular

Question 3

Most pathogens are very _____________ in the ability to infect different species.

Answer 3

Restricted/narrow –> Pathogenesis is host-specific

Question 4

What is virulence?

Answer 4

The degree of damage to the host (or mortality rate in a population) associated with a pathogen.

= Level of harm

Question 5

What is transmission?

Answer 5

The transfer of a pathogen/agent from one individual organism to another = Spread of the disease

In multicellular organisms, we consider this as from an individual to another.

Question 6

What are common means of transmission?

Answer 6

• Contact with the infected individual
• Droplets (fluids)
• Airborne transmission
• Vector (mosquitoes)
• Common vehicles (syringes)

Question 7

What does the seriousness/impact of a disease depend upon?

Answer 7

• Rate of transmission
• Virulence
• Number of susceptibles
• Host recovery rate

NOT all hosts are susceptible.

Question 8

Epidemiological parameters are all influenced by…

Answer 8

The immune system!

Question 9

What is the role of the immune system?

Answer 9

It recognizes foreign bodies and responds with the production of immune cells and proteins.

Question 10

All animals exhibit some _______ immunity.

Answer 10

Innate = Built-in, born with this

Question 11

A subset of animals, vertebrates, also exhibit _____________.

Answer 11

Acquired immunity –> “Memory” in the immune system

Depends on previous exposure

Question 12

What is innate immunity?

Answer 12

• Present before any exposure to pathogens
• Effective from the time of birth
• Involves nonspecific responses to a broad range of pathogens

**Fixed for lifetime

Question 13

What are examples of innate immunity?

Answer 13

External barriers;

Internal cellular and chemical defences.

Question 14

What is acquired immunity?

Answer 14

• Develops after exposure to agents such as microbes, toxins, or other foreign substances
• Involves a very specific response to pathogens

**‘Learned’ response

Question 15

What are examples of acquired immunity?

Answer 15

Antibodies;
“Memory cells” (lymphocytes)

**Veterans of a previous exposure

Question 16

From which mechanisms does innate immunity arise?

Answer 16

• Barrier defences
• Antimicrobial peptides (AMPs) –> Chemical weapons
• Cellular innate defences

Question 17

From which mechanisms does acquired immunity arise?

Answer 17

• Humoral immune response

- Cell-mediated acquired response

Question 18

______________ serve as an example for the basic animal immune system, with no acquired immunity.

Answer 18

Invertebrates.

Example: Cordyceps fungi results in ‘zombie ants’

Question 19

What is epidemiology?

Answer 19

The study of the spread of infectious diseases.

Question 20

What is a synonymous definition to acquired immunity?

Answer 20

Adaptive immunity

Question 21

Innate immunity: Barrier defenses

Answer 21

• Includes skin/exoskeleton and mucous membranes (epithelium)
• Mucus traps and allows for the removal of microbes
• Many body fluids, including saliva, mucus, and tears, are hostile to many microbes

Question 22

What factor of mammalian skin and the digestive system prevent growth of many bacteria?

Answer 22

Low pH

Question 23

What is phagocytosis?

Answer 23

In all animals, the primary means of dealing with foreign substances, including dead cell debris, pathogens, etc.

-Cells eating other things

= Innate immunity, CELLULAR elements

-Engulfing things at a more mechanistic level

Question 24

What are haemocytes?

Answer 24

In invertebrates, a class of patrolling cells that perform phagocytosis, “eating”/engulf foreign particles/cells

-Free-floating in insects

Question 25

How does the phagocytosis process work?

Answer 25

1. Patrol in bloodstream, tissue, lymph of an insect
2. Encounter something foreign
3. Engulf the foreign substance, neutralize them
4. Store the foreign entity in a vacuole, targeted for destruction using a lysosome

Question 26

What are anti-microbial peptides?

Answer 26

Free-floating explosive mines in the lymph of insects, contained within the haemolymph.

Disrupts ‘compatible’ cell walls, membranes of fungi, bacteria and protists.

Built-in chemical defence.

NB. They are FIXED throughout lifetime, innately present in the lymph of insects

Question 27

How do patrolling cells recognize foreign bodies?

Answer 27

TOLL-LIKE RECEPTORS (TLRs).

Located on the surface of the patrolling cells, on the cell membrane –> Detect different pathogens/foreign bodies, facilitating recognition and thus engulfing.

Question 28

What is the key to many cellular defences in vertebrates?

Answer 28

LEUKOCYTES, white blood cells.

Many are part of the innate immune defense
-Patrolling monocytes

Another class within leukocytes is LYMPHOCYTES, central to the acquired immune defence.

Question 29

What can the lymphatic system be compared to?

Answer 29

A highway system to move leukocytes around the body, storing and permitting the maturation/development of these cells.

Question 30

Where do white blood cells typically come from, developmentally?

Answer 30

From stem cells in bone marrow.

They generate two main classes of WBC, lymphoids and myeloids.

Myeloid stem cells –> Erythocytes, neutrophils, basophils, monocytes, platelets, eosinophils

Lymphoid stem cells –> B cells and T cells (lymphocytes)

Question 31

What are the two main types of phagocytic cells in vertebrate animals?

Answer 31

1. MONOCYTES: Group of cellular defenders, such as macrophages, that patrol the tissues/blood, engulf pathogens, signal/initiate the immune response

“Security guards”, call in reinforcement when needed.

2. NEUTROPHILS: Arrive afterwards to engulf and destroy pathogens

“Reinforcement, back-up team”

Question 32

What other cellular innate defenders exist?

Answer 32

• EOSINOPHILS: Release pathogen-destroying enzymes, specialize on multicellular parasites
• NATURAL KILLER CELLS: Detect/destroy abnormal cells by chemical means –> Detonate them
• DENDRITIC CELLS (a second type of monocytes): Messengers stimulated the cells in the acquired immune system.

Question 33

What family of proteins is specific to vertebrate immune systems?

Answer 33

INTERFERONS: They are laid chemical traps, providing innate defense, especially interfering with VIRUSES and helping activate other cells involved in immunity

Question 34

What is the complement system?

Answer 34

A series of specialized proteins, which cause lysis of invading cells, and helping trigger inflammation.

Question 35

How can pathogens avoid destruction?

Answer 35

• Protective coatings on their cell walls
• Modifying surface to avoid recognition

=Counter-weapons, evolution!

-Tolerating lysozyme activity: Even after being broken down following phagocytosis!

Question 36

Are the abilities to avoid host immune systems in most viruses/bacteria innate?

Answer 36

YES! They are fixed in a manner analogous to innate immunity of the host = Genetically determine strain.