Pathogens and host Flashcards
Define ‘pathogenicity’
The capacity of a microorganism to cause an infection.
Describe the concepts of infectivity and virulence
Infectivity= the ability to become established on or within a host Virulence= the ability to cause harmful effects (disease) once established
What is a ‘pathogen’?
an organism that can cause disease
What is ‘commensal’ bacteria?
A commensal is an organism which is part of normal flora (bacteria which are found in or on our bodies on a semi-permanent basis without causing disease) e.g. E.coli in the gut or Staph aureus in the nose
Describe the features of clinical infection.
It’s characterised by signs and symptoms.
Inflammation
pain
pyrexia (raised body temp)
tachycardia (a heart rate that exceeds the normal resting rate)
rigors (shivering, cold)
increased white cell count
Infection can be latent (dormant) or sub-clinical (is nearly or completely asymptomatic)
Describe the features of enterovirus infection with the use of examples
Enterovirus infection= commonly encountered infections especially in infants and children (as they do not yet have immunity from previous exposures to these viruses)
- Example:- poliovirus which can spread person to person and invade a person’s brain or spinal cord causing paralysis.
- Enterovirus infection is spread through saliva that becomes airborne by sneezing and coughing or is present on surfaces or objects that children may touch.
- It moves through the gut and leaves through excretion in the faeces.
- It can go to neuronal and non-neuronal tissues. Neuronal tissues lead to paralysis.
Describe general meaning of humoral immunity
Humoral response allows your body to achieve immunity by encountering pathogens either randomly or on purpose.
- Mostly activated with regards to bacterial infection. Dispatches antibodies. It controls the body’s ‘humors’ or ‘fluids’ e.g blood and lymph. They combat bacteria and viruses moving around interstial space between your cells (extra-cellular).
- Involves B cells that recognise antigens or pathogens that are circulating in the lymph or blood.
-This can happen naturally- if you get flu or chicken pox
or artificially by vaccination
Chain of events in humoral immunity
Humoral= is mediated by antibody molecules that are secreted by plasma cells. it’s located in the extra-cellular space.
Once B-cell has matured they move around blood/lymph
- They detect foreign antigen
- Antigen/ pathogen binds to B-cell and this activates B-cell fully
-Humoral response activated- B-cell produce antibodies that cause the destruction of extracellular microorganisms and prevent the spread of intracellular infections.
- The B cell usually requires a helper T-cell (CD4 T-cells) to activate the B cell. This immune response results in acute inflammation and neutrophilia (increased number of neutrophils in the blood)
Have effector (fight there and then) and memory cells (stay in the body to fight antigen if it returns to the body another time- prepared secondary immune response that is stronger and faster than first)
.
Every individual B-lymphocyte has it’s own…
unique antibodies- each of which is ready to identify and bind to a specific kind of antigen. The more unique antibodies your B-cells have the more likely it is that one will eventually find, bind to and mark a particular antigen.
Cell-mediated (cellular) immunity general terms
An immune response that occurs when all other responses fail–> physical barrier (e.g i.e skin and mucus membrane) fail to keep pathogen out and humoral response wasn’t enough to fight off pathogen
- It does not involve antibodies, but rather involves the activation of phagocytes, T-lymphocytes, antigen-specific cytotoxic T-cells and the release of various cytokines in response to an antigen.
Macrophages ‘present’ the antigen and stimulate T-cells. Cytokines are produced and control the response. Th1 cells activate macrophages to ingest and kill or contain the pathogen
Cytotoxic T-cells CD8 kill infected host cells or foreign cells.
Cell- mediated immunity is important to combat intracellular infection. It results in lymphocytosis- increased number of lymphocytes in the blood.
Function of T-cells
- cause inflammation
- activate macrophages
- regulate much of the immune response
- get other T- cells fired up
Process of cell-mediated immunity
Innate response involves phagocytes (natural killer cells) When they see a suspicious cell they engulf it and kill it. During this attack- the phagocyte breaks the pathogen into lots of tiny molecules that then lie in grooved proteins on outer membrane of phagocyte.
- So phagocytes/macrophages ‘present’ the antigen to T-cells stimulating them.
- Helper T-cells (CD4) raise alarm by releasing a cocktail of chemical messengers called ‘cytokines’.
Some cytokines also go on to activate cytotoxic cells. They roam blood and lymph looking for abnormal cells needing killed.
The cytotoxic T-cell (CD8) releases special enzymes that pierce holes in the antigen’s cell membrane or trigger apoptosis. T-cell then detaches and carries on.
Two types of T-cells
Helper T-cells= can’t kill themselves but can activate cells that do- call the shots for this immune response
cytotoxic- carry out the killing of aberrant cells.
AIDs
caused by human immunodeficiency virus that specifically attacks helper T-cells and without these there wouldn’t be much of a humoral response, as helper T cells present antigen to B cells and activate the to produce humoral response (antibody production by plasma) and also natural killer cells and macrophages wouldn’t be activated
Importance of cell mediated immunity in infection
Combat intra-cellular infection
Viral entry sites- 3
Conjunctiva Arthropod Capillaries Skin respiratory tract alimentary tract (digestive) urinogenital tract
Features of pathogenic toxins and their effects
A toxin is a poisonous substance- acts as an antigen in the body.
Exotoxins are released extracellularly by the micro-organism
Enterotoxins are exotoxins which act on the GI tract i.e ones that come into the body through oral or airborne routes
Endotoxin is a lipopolysaccharide which is structurally part of the Gram negative cell wall. Induces severe uncontrolled host response. Cytokine production. Fever, rigors etc
Tetanus
Exotoxin- Clostridium tetani causes an infection in a dirty wound and produces toxins.
Tetanus results in uncontrolled muscle spasm due to inhibition of neurotransmitter release at N.M.J
Antigenically modified toxin used for immunisation
Enterotoxin: cholera
Spread by unsafe water/food.
Colonises in small intestine and produces enterotoxins.
Increase cAMP levels
Massive passive outflow of H20- death by dehydration
treated by rehydration
What are the harmful effects of pathogens caused by?
The bacterium, the host response or both
Virus pathogenic mechanisms i.e how they affect the body
- Cell destruction following virus infection
- Virus-induced changes to cellular gene expression
- Cellular transformation by tumour viruses
- immunopathogenic disease.
Influenza A virus
Airborne
virus infects cells of the respiratory tract
destruction of respiratory epithelium
Altered cytokine expression leading to fever.
Innate immunity
Innate immunity is already present in the body. Phagocytic cells ingest foreign bodies
Acquired (adaptive) immunity
Specific response to antigen concerned. Immunological memory created. Humoral (antibody) and cellular (T cells) aspects
Types of phagocytic cells
macrophages, neutrophils, basophils, eisonophils, dendritic cells, monocytes (precursor macrophages) and mast cell IMPORTANT IN INNATE
What is an acute viral infection
Rapid onset of disease, a relatively brief period of symptoms, and resolution within days. Elimination of infection by the host immune system. Example:- influenza A- respiratory infection
Define colonisation.
The development of a bacterial infection on an individual, as demonstrated by a positive culture.
- The infected person may have no signs or symptoms of infection while still having the potential to infect others.
Define infection
The invasion and multiplication of microorganisms such as bacteria, viruses, and parasites that are not normally present within the body
Asymptomatic
If a patient is a carrier for a disease or infection but experiences no symptoms.
Natural immunisation
exposure/infection
Artificial immunisation
Vaccination
What is pathogenicity dependent on?
Infectivity
Virulence
How can infectivity occur?
Attachment to host
entry through viral site
Acid resistance in stomach after entering body
What is virulence determined by?
virulence factors which are genetically microbial components such as:-
Invasiveness
toxin production
evasion of immune system
Virulence factors are specific to the strain not the species.
What structural role do endotoxins have?
Part of the gram negative cell wall. (Lipopolysaccharides)
Structure of an endotoxin?
Lipid A
Oligosaccharide
specific polysaccharide chain
What do endotoxins induce?
A severe uncontrolled host response which causes cytokine production fevor rigors hypotension (low bp) tachycarda (increased heart rate) collapse
What is antigenic drift?
Minor changes in the genes of viruses (natural mutations)
it occurs gradually over time to produce antigenic variants
Antigenic shift?
Abrupt major changes in the virus antigenic structure
Some examples of enteroviruses?
single stranded RNA 'entero' means intestinal Poliovirus aseptic meningitis or 'viral meningitis' - caused by virus not bacteria. Viral is more common. myocarditis pancreatitis
Virus and cancer
Both DNA and RNA viruses have been shown to be capable of causing cancer in humans
Virus affects host’s genome and have potential to turn healthy cells into cancerous ones.
Name the host defence mechanisms
Innate immunity
phagocytic mechanisms
antibody and complement
cell mediated immunity
Opsonisation?
Organism coated with antibody or complement. marking so phagocytic cells identify them
Phagocytic cell has receptors for both
-Efficiency of phagocytosis greatly improved
Types of lymphocytes (leukocyte)
IMPORTANT IN ADAPTIVE
T and B cells, Natural killer cells
