Pathogens and Hosts Flashcards
Describe some host defence mechanisms
- Innate immunity - Phagocytic mechanisms
* Acquired immunity - Antibody and complement (cell mediated immunity)
Describe some physical barriers to infection
Skin
Gastric acid
Muco-ciliary escalator
Describe the process of phagocytosis
- Phagocytosis (ingestion) of a particle
- Organism held in phagosome
- Fusion with lysosome
- Phagolysosomeis formed
- Intra-cellular killing
Describe briefly the process and function of opsonisation
- Organism coated with antibody or complement
- Phagocytic cell has receptors for both
- Efficiency of phagocytosis greatly improved
What type of reactions are involved in the adaptive immune response?
Specific
Humoral (antibodies)
Cell mediated
What antibody is typically found in primary infections?
IgM
What antibody is typically found in secondary infections?
IgG
What antibody is typically found in mucosal immunity?
IgA
What antibody is typically found in allergy or parasite infections?
IgE
Briefly describe the complement system
The complement system is a part of the innate immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promotes inflammation, and attacks the pathogen’s plasma membrane. The complement system consists of a number of small proteins found in the blood, in general synthesized by the liver, and normally circulating as inactive precursors (pro-proteins). When stimulated by one of several triggers, proteases in the system cleave specific proteins to release cytokines and initiate an amplifying cascade of further cleavages. The end result of this complement activation or complement fixation cascade is stimulation of phagocytes to clear foreign and damaged material, proxy inflammation to attract additional phagocytes, and activation of the cell-killing membrane attack complex.
How many complement proteins are there?
~20
What is the use of antibodies in infections?
- Neutralises bacterial toxins (e.g. tetanus, diphtheria)
- Neutralises viruses in viraemic stage
- Prevents adherence of microorganisms
- Opsonises capsulate organisms (Strep pneumoniae, Haemophilus influenzae)
- Useful means of diagnosis (serology)
What are the three main functions of the complement system?
Phagocytosis – by opsonizing antigens. C3b has most important opsonizing activity
Inflammation – by attracting macrophages and neutrophils
Membrane attack – by rupturing membranes of foreign cells and gram negative bacteria
Whats the primary functions of the two CD4+ T cell subtypes?
Both release specific cytokines related to the type of infection to cause either:
– Th1 - cell mediated immunity (macrophages, CD8+)
– Th2 - control B cell antibody response
Whats the importance of cell mediated immunity?
- Important in the clearance of:
- Most viral infections and fungal infection
- Intra-cellular infection
- Lymphocytosis - increased WBC during infection
Describe some signs of clinical infection
– Inflammation can be cause by infection or AI ailment
– Pain
– Pyrexia (fever)
– Tachycardia (increased HR)
– Rigors (whole body shaking uncontrollably >5mins, body desperately trying to create a fever and fight infection)
– Increased white cell count
– Increased C reactive protein (CRP) – marker of inflammation
– Note that infection can also be latent or sub-clinical
Whats the difference between a pathogen and a commensal?
- A pathogen is an organism which can cause disease
- A commensal is an organism which is part of normal flora e.g. E. coli in the gut, Staph aureus in the nose, axilla
- The distinction between these is not always clear e.g. during immunosuppression, antibiotic treatment or infection, opportunistic infections can occur when commensals take over
How can you tell if an organism is a pathogen or a commensal?
o Koch’s Postulates - organism must be found in all cases of the disease, be able to be cultured outside the body for several generations and should reproduce the disease on inoculation
o Sterile vs. non-sterile sites - knowledge of normal flora for site, the organism’s pathogenicity and its clinical context
What is Koch’s postulate?
Koch’s Postulates - used to see if an organism is pathogenic or a commensal. The organism must be found in all cases of the disease, be able to be cultured outside the body for several generations and should reproduce the disease on inoculation
Describe some sterile sites in the human body
Blood Amniotic fluid Synpvial fluid Bone and bone marrow Cerebrospinal fluid
What are the two requirements of an organism needed to be pathogenic and cause an infection?
Infectivity - Ability to become established
Virulence - Ability to cause harmful effects once established
Describe some factors which can increase infectivity of an organism, giving some examples
Attachment mechanisms - E.Coli and P-fimbrae
Acid resistance - helicobacter pylori and urease
Describe some virulence factors
Virulence factors - genetically determined microbial components that can influence:
• Invasiveness
• Toxin production
• Evasion of immune system
Describe some pathologies caused by Streptococcus pyogenes and how it plays a role in its invasiveness
Streptococcus pyogenes (Group A streptococci) causes: Necrotising fasciitis Cellulitis Connective tissue breakdown by enzymes: • Hyaluronidase • Collagenase Fibrinolysis • Streptokinase
