Microbiology A1 - Natural & Innate Immunity Flashcards
Barriers to infection - What are they? (4)
- Skin
- Mucociliary clearance
- Secreted antibacterial compounds
- The urinary tract
Barriers to infection - Skin
How does it prevent infection?
- Anatomical barrier physically preventing invasion of microorganisms.
- Chemical barrier providing unfavourable conditions for most microorganisms to survive due to:
- Free fatty acids produced by the sebaceous glands and skin flora
- Lactic acid in the sweat
- Low pH
- Relatively dry environment
Barriers to infection - Mucociliary Clearance
How does it prevent infection?
- Present in respiratory tract
- Mucus is produced by goblet cells and submucosal mucus-secreting glands on cilated columnar epithelial cells.
- Cilia beat to move mucus to the oropharynx to be swallowed
- Smaller particles reaching the alveoli are phagocytosed by macrophages and transported to the mucus stream
- Nasal secretions contain antibodies, lysozyme and interferon
- This can be disrupted by pollutants (e.g. cigarette smoke) and microorganisms (e.g. rhinovirus) which attach to receptors on epithelial cells, damaging them and disrupting cilia motility
Barriers to Infection - Secreted Antibacterial Compounds
5
Lysozyme - Found in tears, saliva and mucus. Enzyme that attacks peptidoglycans in bacterial cell walls.
Lactic acid - Found on the skin and vagina
Free fatty acids - Found on the skin
Hydrochloric acid - Found in the stomach
Lactoperoxidase - Found in breast milk and saliva. Inhibitory to many microorganisms.
Barriers to Infection - The Urinary Tract
How does it prevent infection?
Urine flushes out microorganisms and is acidic.
Normal Bacterial Flora
What are they and how do they work?
How do they inhibit pathogens?
What could negatively affect them and what might this lead to?
They are bacteria that live on and in us all the time. They out compete other bacteria that may invade.
- Some flora naturally produce antibiotics and bacteriocins, inhibiting the spread of pathogens.
- Natural Flora stimulate the production of IgA (an important surface antibody), protecting us from deeper invasion
Phagocytes and Complement
Complement
What is the ‘complement system’ and what does it result in?
- The complement system is a group of about 20 serum proteins that work in a cascade containing an amplification loop in response to infection
- It has 3 pathways: Alternative, Classical, Letcin.
- It leads to the formation of the membrane attack complex, which lyses bacteria by forming water-permeable channels within their cell-membranes.
- Migrates phagocytes to the site of infection and increases vascular permeability.
Phagocytes and Complement
Complement
What effects do congenital defects of C3 and C5-8 have?
Affect opsonization and the membrane attack complex, making patients susceptible to encapsulated organisms such as: Streptococcus pneumoniae, Nisseria menigitidis, Gonococci Haemophilus influenzae.
Which skin conditions increase the risk of infection
burns
eczema
psoriasis
What does systemic antibiotic therapy do to normal flora (2 examples)
Removes normal flora, allowing other opportunistic pathogens to colonise
- Clostridium Difficile leading to enterocolitis
- Candida Albicans leading to oral thrush
What are the two main granulocytes in phagocytosis
Neutrophils and macrophages
What do neutrophils do in phagocytosis?
The most abundant granulocyte. They are attracted to the infection site and ingest around where the microbe is adhered, forming a phagosome. They release granules containing microbiocidal molecules. High neutrophil count indicates bacterial infection. Injury or infection stimulates chemotactic factor which allows them into the tissues.
What do macrophages do in phagocytosis?
When they phagocytose they trigger an acute phase response in response to bacterial cell walls. They release interleukins, TNF and Chemokines.
What do opsonins do?
They are co-factors that bind and coat microbes enhancing phagocyte binding.
Eg: C3b and CRP
What role does the spleen play in normal macrophage function?
The spleen removes old blood cells, helps in antigen recognition and stores monocytes and macrophages. After a splenectomy patients need to be immunised against encapsulated Haemophilus influenza and Strep pneumonia, and need prophylactic antibiotics.
