The Immune System: Pathogens, Infection and Defenses Flashcards
What is a pathogen?
An agent that potentially infects and damages host cells.
What are examples of potential agents within infectious diseases, in order of size/complexity?
- Prions: Non-living, but have life-like properties
- Viruses
- Bacteria
- Protists
- Fungi: Eukaryotes, multicellular
Most pathogens are very _____________ in the ability to infect different species.
Restricted/narrow –> Pathogenesis is host-specific
What is virulence?
The degree of damage to the host (or mortality rate in a population) associated with a pathogen.
= Level of harm
What is transmission?
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.
What are common means of transmission?
- Contact with the infected individual
- Droplets (fluids)
- Airborne transmission
- Vector (mosquitoes)
- Common vehicles (syringes)
What does the seriousness/impact of a disease depend upon?
- Rate of transmission
- Virulence
- Number of susceptibles
- Host recovery rate
NOT all hosts are susceptible.
Epidemiological parameters are all influenced by…
The immune system!
What is the role of the immune system?
It recognizes foreign bodies and responds with the production of immune cells and proteins.
All animals exhibit some _______ immunity.
Innate = Built-in, born with this
A subset of animals, vertebrates, also exhibit _____________.
Acquired immunity –> “Memory” in the immune system
Depends on previous exposure
What is innate immunity?
- Present before any exposure to pathogens
- Effective from the time of birth
- Involves nonspecific responses to a broad range of pathogens
**Fixed for lifetime
What are examples of innate immunity?
External barriers;
Internal cellular and chemical defences.
What is acquired immunity?
- Develops after exposure to agents such as microbes, toxins, or other foreign substances
- Involves a very specific response to pathogens
**‘Learned’ response
What are examples of acquired immunity?
Antibodies;
“Memory cells” (lymphocytes)
**Veterans of a previous exposure
From which mechanisms does innate immunity arise?
- Barrier defences
- Antimicrobial peptides (AMPs) –> Chemical weapons
- Cellular innate defences
From which mechanisms does acquired immunity arise?
- Humoral immune response
- Cell-mediated acquired response
______________ serve as an example for the basic animal immune system, with no acquired immunity.
Invertebrates.
Example: Cordyceps fungi results in ‘zombie ants’
What is epidemiology?
The study of the spread of infectious diseases.
What is a synonymous definition to acquired immunity?
Adaptive immunity
Innate immunity: Barrier defenses
- 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
What factor of mammalian skin and the digestive system prevent growth of many bacteria?
Low pH
What is phagocytosis?
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
What are haemocytes?
In invertebrates, a class of patrolling cells that perform phagocytosis, “eating”/engulf foreign particles/cells
-Free-floating in insects
How does the phagocytosis process work?
- Patrol in bloodstream, tissue, lymph of an insect
- Encounter something foreign
- Engulf the foreign substance, neutralize them
- Store the foreign entity in a vacuole, targeted for destruction using a lysosome
What are anti-microbial peptides?
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
How do patrolling cells recognize foreign bodies?
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.
What is the key to many cellular defences in vertebrates?
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.
What can the lymphatic system be compared to?
A highway system to move leukocytes around the body, storing and permitting the maturation/development of these cells.
Where do white blood cells typically come from, developmentally?
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)
What are the two main types of phagocytic cells in vertebrate animals?
- 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.
- NEUTROPHILS: Arrive afterwards to engulf and destroy pathogens
“Reinforcement, back-up team”
What other cellular innate defenders exist?
- 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.
What family of proteins is specific to vertebrate immune systems?
INTERFERONS: They are laid chemical traps, providing innate defense, especially interfering with VIRUSES and helping activate other cells involved in immunity
What is the complement system?
A series of specialized proteins, which cause lysis of invading cells, and helping trigger inflammation.
How can pathogens avoid destruction?
- Protective coatings on their cell walls
- Modifying surface to avoid recognition
=Counter-weapons, evolution!
-Tolerating lysozyme activity: Even after being broken down following phagocytosis!
Are the abilities to avoid host immune systems in most viruses/bacteria innate?
YES! They are fixed in a manner analogous to innate immunity of the host = Genetically determine strain.
