2. Infection & Immunity Flashcards
Non-specific immune response
- All animals
- Immediate & the same for all pathogens
- Physical barriers (e.g. skin, mucous membranes)
- Chemical barriers (e.g. stomach acid)
- Phagocytosis
Specific immune response
- Only vertebrates
- Slower & specific to each pathogen
- Cell-mediated response (T cells)
- Humoral response (B cells)
Non-specific response in respiratory system
- Trachea, bronchi & bronchioles secrete mucus to trap microorganisms
- Cilia move the mucus to the pharynx so we swallow them
- They are then digested in our stomach & intestines
Non-specific response in skin
- Multilayer structure: difficult for microorganisms to penetrate
- Outside layers are dead cells filled with keratin (indigestible protein)
- Waxy sebum makes our skin supple & its fatty acids are toxic to many microorganisms
Non-specific response in the intestines
Normal flora enzymes & low stomach pH destroy pathogens
100s of species of commensal/mutualistic bacteria inhabit our gut and out-compete pathogenic microorganisms
Antigen
Any part of an organism (often a protein on the surface of a cell) that is recognised as foreign by our immune system, therefore capable of triggering an immune response
4 things our immune system can identify by their antigens
- Pathogens
- Cells from other organisms of the same species
- Cancerous cells
- Toxins
Cell recognition: self vs non-self
A self marker (MHC) labels the body’s cells as a “friend” and are tolerated by the immune system
Why are transplant organs often taken from relatives?
Antigens are genetically controlled - close relatives have more similar antigens so decreased risk of organ rejection
What are antigens recognised by?
Lymphocytes, which bind to and detect the characteristic shape of an exposed portion (epitope)
Cells have different genetically-determined _____________
proteins and glycoproteins on their surface
Glycoproteins that identify cells are called…
Major Histocompatibility Complex (MHC) proteins
4 steps of phagocytosis
- Phagocyte detects and moves towards chemicals released from the pathogen
- Phagocytes surround & engulf the pathogen into a vesicle. The vesicle FUSES with the phagosome releasing the pathogen (endocytosis)
- Lysosomes also fuse with the phagosome and release lysozymes to destroy the pathogen
- Hydrolysed products are absorbed by the phagocyte
What are the 2 types of phagocytes?
Neutrophils & macrophages
They are also known as phagocytic leucocytes
Are most WBCs neutrophils or macrophages?
Neutrophils (70%)
What is a characteristic feature of a neutrophil?
Three-lobed nucleus
Why are B cells called B cells?
Produced in bone marrow and stay there
Why are T cells called T cells?
Produced in bone marrow but move to thymus gland to mature
What proteins do T cells have?
Receptor proteins that can detect antigens and form antigen-receptor complexes
Process of cell-mediated immunity
- Pathogens invade body cells & taken in by phagocytes
- Phagocyte places antigens from pathogen on its cell-surface membrane (antigen presentation)
- Receptors on a specific helper T cell fit exactly onto these antigens
- This attachment activates the T cell to divide rapidly by mitosis (clonal expansion) and form a clone of genetically identical T cells
What can the cloned T cells do after clonal expansion?
- Develop into memory cells that enable a rapid response to future infections by the same pathogen
- Stimulate phagocytes to engulf pathogens by phagocytosis
- Stimulate B cells to divide & secrete antibodies
- Activate cytotoxic T cells
How do cytotoxic T cells kill infected cells?
They produce a protein called perforin that makes holes in the cell-surface membrane. This makes it freely permeable to all substances, so the cell dies as a result.
Tc cells are most effective against viruses because viruses replicate inside cells.
What does humoral immunity mean?
Immunity using antibodies dissolved in tissue fluid/plasma
What do B lymphocytes do?
Display & secrete antibodies that can detect antigens and form antigen-antibody complexes
Process of humoral response
- Invading pathogen engulfed by phagocytes
- B-cell’s antibodies bind to pathogen’s antigens to form antibody-antigen complexes
- B cell can activate helper T cells (which tell B cells to divide/trigger cell-mediated response) OR clone by mitosis
- If B cells clone by mitosis, they become either plasma B cells or memory B cells
What do plasma B cells do?
Secrete antibodies which attach to antigens on the pathogen and destroy it (primary response)
What do memory B cells do?
Circulate in blood & tissue fluid in readiness to respond to a future infection by the same pathogen/antigen by dividing and developing into plasma cells that produce antibodies (secondary response)
How are antigens presented on B cells?
They enter the B cell via endocytosis and get presented on its surface (processed).
Antibodies
Proteins with specific binding sites synthesised by B cells
Where are disulphide bridges found in an antigen?
Between the two bigger/inner branches
Structure of an antibody
- Composed of 4 polypeptide chains (2 heavy & 2 light), joined by peptide/disulphide bonds
- Y-shaped: the stem is the constant region, the ends of the arms are variable regions that bind to the antigen
- Unique binding site that fits to a specific antigen to form an antigen-antibody complex
What key process do antibodies carry out?
Agglutination: linking cell-bound antigens together, causing clumping & targeting them for destruction by phagocytes
How else do antibodies stop disease aside from agglutination?
- Prevent viruses & bacteria from infecting cells
2. Binding to free toxin proteins
Primary vs secondary response
Antibodies produced faster & in higher concentrations in secondary response
Features of primary response
- Relatively few initial specific T & B cells
- Relatively few clones produced
- Symptoms of disease visible
Features of secondary response
- Memory cells present & ready to respond to a second infection
- Much faster response: memory T cells divide into Tc cells, memory B cells divide into plasma cells
- Many more T and B cells produced: much STRONGER response
- Pathogen destroyed before it can cause symptoms
Does cell-mediated immunity involve antibodies?
NO
Which comes first: cell-mediated or humoral immunity?
Cell-mediated
4 things a vaccine might contain
- Attenuated pathogen
- Modified toxins (toxoids)
- Antigen-bearing fragment of pathogen
- Genetically engineered DNA
What does attenuated mean?
Reduced ability to cause infection or inactivated pathogen
2 reasons for pathogens having different antigens
- Mutations
2. Horizontal transfer of plasmids containing new genes (conjugation)
The length of time someone is immune to infection following a vaccination depends on…
how long the memory cells survive for
Natural active immunity
Producing antibodies in response to exposure to pathogenic infection
Artificial active immunity
Producing antibodies in response to the controlled exposure to an attenuated pathogen (i.e. vaccination)
Natural passive immunity
Receiving antibodies from another organism (e.g. to the foetus via the colostrum or a newborn via breastmilk)
Artificial passive immunity
Receiving manufactured antibodies via external delivery (e.g. blood transfusions of monoclonal antibodies)
Theory of herd immunity
Provided a large enough population are immune to a particular contagious disease, the likelihood of the causative pathogen being transmitted to a member of the population who is not immune is negligible.
Valid reasons for not being vaccinated
- Allergic
2. Immunocompromised (chemotherapy/immunosuppressants)
Pros of vaccine
- Person is protected against diseases that could otherwise kill or disable it
- Society benefits as the potential pool of infection is reduced with every vaccinated child through herd immunity, protecting children who cannot be vaccinated
- Cost of treating serious diseases & caring for those left permanently damaged by them is kept to a minimum for a relatively small financial outlay
Cons of vaccine
- Some attenuated vaccines cultured in eggs: some children are allergic to eggs
- Extreme immune response (becoming very ill after the vaccine)
- Benefit of society rather than direct benefit of the child (e.g. rubella for boys)
General differences between active & passive immunity
- Active involves memory cells; passive doesn’t
- . Active involves production of antibodies by plasma/memory cells
- Passive involves antibody introduced into body from outside source
- Active long term because antibody produced in response to antigen
- Passive short term because the antibody is broken down
- Active takes time to develop, passive is fast acting
