why are viruses not considered cells?
as they have no cytoplasm, cell membrane etc…
why are viruses not considered living organisms
as they have no metabolism and can only reproduce inside host cells and their replication depends on metabolism of host cells
describe structure of viruses
1- strand of linear DNA or RNA
2-protein coat called capsid made of capsomeres
3-external envelope made of lipids and proteins
4-surface proteins that are complementary on surface proteins on host cells and bind to them helping virus enter host cell
how to produce new viral particles
1- replication; the genetic material of viruses replicate inside host cells
2-protein synthesis; the virus uses host cell energy, enzymes and nucleotides to form mrna , then mrna is incorporated into host cell ribosomes where its translated to form protein coat
3-assembly; the new genetic material is incorporated into new protein coat forming large numbers of viral particles, they cause lysis of infected cells and burst out of the cells infecting nearby cells
describe mechanism of infection of virus
1-the viral surface proteins bind to a specific type of receptor on host cell surface
2- the virus injects its core (DNA\rna + or - enzymes
that’s why each virus only binds to a specific type of cells
eg; influenza virus only binds to respiratory cells and not digestive system cells
describe lysogenic pathway
1-in DNA viruses; the viral DNA is incorporated into DNA of host cell using viral integrase enzyme
2- in RNA enzymes; an RNA strand is used as a template to synthesize DNA this process is called reverse transcription and is catalyzed by the enzyme reverse transcriptase then the viral DNA is incorporated into DNA of host cell using viral integrase enzyme
- the viral genetic material only replicates with replication of host cells - the viral stays dormant inside the cell not causing any symptoms
-a virus may suddenly shift from lysogenic to lytic pathway
how do antiviral drugs slow down lysis process
1- they block receptors of host cells so viral particles cannot bind
2-so the virus cant inject its core into the host cell and cant replicate itself
3-this prevents lysis of host cells
-some antivirals work by blocking enzymes responsible for its replication
what are the routes of infection of diseases
1-broken skin barrier
2-openings of the body (mouth, eyes etc.)
3- bacteria flora may turn pathogenic
what are patterns of disease spread
1-endemic; disease spread at constant low rate in a certain area
2-epidemic; widespread of disease at a high rate to nearby areas
3-pandemic; worldwide spread of disease
what are the modes of infection
1-inhalation (airborne); coughing or sneezing from an infected person , droplets containing pathogens are inhaled by a healthy person
2-ingestion; of contaminated foods or drinks which causes diseases such as salmonella
3-vectors; living organisms transmit diseases from an infected person to a healthy person such as mosquitos with malaria
4- fomites; objects that carry infections such as hospital towels or bedding and blood transfusion or sharing needles
5- direct contact; such as spread of skin infections and sexually transmitted disease such as gonorrhea
what’s the difference between non specific immune mechanisms and specific immune response
non specific doesn’t depend on a specific type of pathogens and doesn’t change with repeated infections while specific depends on a specific type of pathogens and improves with repeated infections
state and explain physical barriers to infection
1-intact skin; it consists of keratin which is an indigestible protein that acts as a barrier and prevents entry of pathogens
2- bacteria flora; is a harmless normal bacteria which outcompetes pathogenic bacteria for space and nutrition and may release chemicals which make medium unsuitable for pathogenic bacteria and they also may destroy them
3- lysozymes ; they digest bacterial cell wall killing pathogenic bacteria, they are secreted by skin in sebum, lacrimal glands in eyes, respiratory epithelium and phagocytes
4-respiratory epithelium; contains mucus which traps pathogens and cilia which wafts this mucus away to the throat
5-stomach; contains -stomach acid which kills bacteria -bacteria flora which outcompetes pathogenic bacteria for space and nutrition -pepsin which digests proteins -anaerobic conditions which kill aerobic bacteria
state and explain how inflammation happens
entry of a pathogen or a foreign body causes mast cells to rupture (more blood) releasing a chemical called histamine which acts as a local vasodilator which allows more blood containing phagocytes that engulf bacteria to flow into arterioles and increases permeability of capillaries
what are the characteristics of inflammation
1- redness (due to vasodilation as there’s more blood)
2- hotness (better enzymatic activity)
3-oedema (swelling)
how does a fever happen
1-the body’s immune response to infection may lead to fever
2-pathogens cant enter the brain as they cannot pass the blood brain barrier, however they release toxins that are small enough to pass which may affect the thermoregulatory center of the hypothalamus affecting heat loss mechanisms
what is the importance of fever
1-temperature higher than 37 degrees is not optimum for pathogens so it may denature their enzymes or destroy them
2-our immune systems activity is better at higher temperature
what are the hazards of fever
1- if temperature exceeds a certain limit our enzymes may denature
2-excessive sweating may cause dehydration and loss of important ions
what is the simplified mechanism of pathogens
1-they engulf bacteria
2-they digest it using hydrolytic enzymes
3-they destroy it
define MHC
glycoprotein receptors on the surface of the body cells that can be recognized by the immune system as self (they are unique for each person)
define foreign antigen
glycoprotein on the surface of pathogens that can be recognized by the immune system as non-self
what are antibodies and their function
1-they are proteins produced by plasma cells
2-they are complementary to foreign antigens
3-they bind to antigens forming antigen-antibody complexes
describe the structure of antibodies
they consist of 4 polypeptide chains;
1- 2 heavy chains joined together by disulfide bonds
2- 2 light chains
- constant regions are made up of heavy chains only
-variable regions are made up of both heavy and light chains
what is the mechanism of antibodies
-they bind to antigens forming antigen-antibody complex
1- agglutination (immobilization and clumping) : they clump together for easy phagocytosis
2-opsonization (marking) : attracting phagocytes for faster phagocytosis
3- toxic neutralization; they bind to toxins of bacteria neutralizing them
how does t helper cell activation occur
-t helper cell activation must occur before antibody mediated or cell mediated immune response
1-macrophages engulf bacteria and secrete lysozymes to digest it
2-they extract the antibody (processing) to be displayed on the MHC of macrophages
3-the macrophage now becomes an antigen presenting cell
4- T helper cells with CD-4 receptors bind to antigens on MHC of macrophages
5- then they divide by mitosis into effector t helper cells and memory t helper cells
