Innate immune response;
Complement proteins directly puncture the bacterial cell wall and membrane and make it susceptible to osmotic lysis, opsonised bacteria, which are coated with antibodies and complement proteins are phagocytosed, neutrophils are particularly active during bacterial infections. Peripheral blood levels of neutrophils increase in response to increased bone marrow production. This is especially common with gram- positive bacterial infections. Extreme infections may cause neutropenia due to increased demand for neutrophils outstripping the ability of the bone marrow to supply them and monocytosis may occur, particularly in the resolution phase of a bacterial infection.
Adaptive immune responses;
Intracellular: Cell-mediated immunity is launched against intracellular bacteria (such as Salmonella), which cannot be accessed by complement or antibodies. Infected macrophages present bacterial proteins on their cell surface using MHCII receptors. Helper T cells detect these and release interferon, which stimulates the macrophage to digest the bacterium-infected macrophages
Extracellular: Extracellular bacterial infections (i.e. Staphylococcus aureus) are the most frequent of all. In such cases, the protection mechanisms are mainly related to the host's natural barriers, other innate immune responses and antibody production by the adaptive immune system
Once the virus is inside the cell, the host's immune system has no access to the virus, therefore the infected cells use MHCI molecules on their cell membrane to present pieces of viral proteins to the outside of the cell.
Cytotoxic T cells recognise specific virally infected cells and only T cells with the specific T cell receptors for that particular virus are activated. The cytotoxic T cell then release cytotoxic factors that kill the virally infected cell. They also produce cytokines that prevent the replication of viruses inside the infected cell.
Natural killer cells (phagocytes) can detect if cells with fewer than normal MHCI receptors are present and release cytotoxic factors, killing them as they would a virally infected cell.
Full blood counts are unpredictable in viral infections; the white cell count may be reduced due to the suppression of bone marrow production of white blood cells.
Pathogens factors pose unique challenges for the adaptive and innate immune response: many have multiple life stages (parasitic worms), many worms can infest a number of hosts as they develop, changing dramatically before moving to the next host, chronic infestation can lead to pathological changes, parasites may modulate their surface structure to avoid recognition by the host, molecular cross-talk occurs between helminths and the mammalian immune system.
Host factors: full blood counts usually show eosinophilia, Th2 cytokines such as interleukin are increased, initial increase in T cell reaction dampens as the infestation becomes chronic, memory protects the host from new infections while the old infestation continues, antiparasitic IgE levels correlate with resistance to new infestation in the host, Th2 response gene in some families help confer resistance, eosinophils kill opsonised helminths.
Ebola virus disease (EVD) is a server, fatal disease in humans
Fruit bats are the natural host of EVD and is initially transmitted to humans from an infected animal and continues to spread through direct human-to-human contact.
Factors that affect transmission: virulence of the virus strain, population density, population mobility in infected area, host exposure and susceptibility, cultural beliefs and behaviour practices, public health infrastructure.
EVD initially infects the cells of the dendritic cells of the immune system, preventing an immune response. It then replicates on mass and infects the cells of multiple organs resulting in cell death. This leads to the release of cytokines, which thins the blood vessel walls causing them to leak blood. Blood pressure drops significantly as well as body temperature, causing the infected person to go into shock and die.
Incubation period: 2 to 21 days
Initial symptoms: fever, fatigue, muscle pain, headache and sore throat
Severe symptoms: vomiting, diarrhoea, rash, impaired kidney and liver function, internal and external bleeding
Can persist for more than 9 months after recovery in the testicles, eyes, CNS and placenta and milk of pregnant women
Prevent and control: avoid human-to-human transmission through wearing personal protective equipment, ensure good personal hygiene and refrain from unprotected sex.
Avoid animal-to-human transmission by cooking meat thoroughly, surveillance of known hot spots, contact tracing laboratory testing and safe burials, social mobilisation and quarantine.
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