What is Immunity?
Jan 19, 2016
What is Immunity?
Immunity and Disease
■ Immunity is the ability of organisms to defend themselves against pathogens and toxins.
■ Infection is the harmful colonisation of an organism by another species.
Pathogens
■ A pathogen is an organisms that causes disease.
■ Many pathogens are microbes
■ Pathogens infect and cause harm
(Natural)
(Natural)
General Defence System
Non-specific Defence: measures to prevent entry of all pathogens
Skin
■ Physical barrier of dead cells (epidermis)
■ Antimicrobial substances in sebum■ Acidic nature of sebum and sweat■ Blood clotting■ Sebum moisturises which prevents
cracking of skin (would allow entry).
Digestive system■Antimicrobial in saliva■HCl in stomach kills bacteria■Symbiotic bacteria in colon
protect against pathogenic bacteria
Breathing System
■Hairs and mucus in nasal passage
■Mucus in bronchi contains antimicrobial
■Cilia move mucus containing trapped foreign bodies up and out of respiratory tract.
Phagocytic White Blood Cells
■ Phagocytes are special defence cells that feed like amoeba
■ They engulf and digest material (e.g. a virus or bacterium) – phagocytosis
■ Macrophages are large aggressive phagocytes.
■ There are many phagocytes present in the lymph nodes.
Fever■ Raised temperature following
infection■ Increases ability to defeat
pathogensInterferon■ Produced by virus-infected cells■ Increases resistance of
neighbouring non-infected cells
Specific Defence System
How our body can tell the difference between self and non-self
Antibodies and Antigens
■ Antigens are non-self chemicals that stimulate the production of antibodies
■ Antibodies are specific proteins formed in response to the presence of an antigen – antibody binds specifically to the
antigen
How an antibody operates/works?
Deactivation of a bacterium by an antibody.
Monocytes
■ Make up 75% of the white blood cells
■ Some leave the blood and enter tissue becoming macrophages – a type of phagocyte
■ Once they have engulfed antigen, they “present” the antigen on their surface
■ Lymphocytes monitor monocytes for any signs of foreign antigens.
■ Monocyte action is accelerated by Helper T-cells
Phagocytosis
Engulfing germ
Germ
MonocyteGerm
Lymphocytes
■ Specialised white blood cells■ Able to distinguish between self and
non self■ Produced in the bone marrow■ Huge numbers in lymphoid tissue
– spleen, tonsils, adenoids, thymus, intestinal wall
LYMPHATIC SYSTEM
Lymphatic Vessels
One-Way Valves
Tonsils
Thymus
Spleen
Immunity
Active Induced Immunity■ Lymphocytes are activated by antigens
on the surface of pathogens■ Antibodies are produced by the
lymphocytes
Natural active immunity - acquired due to infectionArtificial active immunity – vaccination
■ Takes time for enough B and T cells to be produced to mount an effective response.
Passive Induced Immunity
■ B and T cells are not activated and plasma cells have not produced antibodies.
■ The antigen doesn’t have to be encountered for the body to make the antibodies.
■ Antibodies appear immediately in blood but protection is only temporary.
Artifical Passive ImmunityArtificial passive immunity■ Used when a very rapid immune
response is needed e.g. after infection with tetanus.
■ Human antibodies are injected. In the case of tetanus these are antitoxin antibodies.
■ Antibodies come from blood donors who have recently had the tetanus vaccination.
■ Only provides short term protection as abs destroyed by phagocytes in spleen and liver.
Natural Passive Immunity
Natural Passive immunity■ A mother’s antibodies pass across
the placenta to the foetus and remain for several months.
■ Colostrum (the first breast milk) contains lots of IgA which remain on surface of the baby’s gut wall and pass into blood
Vaccination■ A vaccine is a preparation which gives artificial
active immunity to a pathogen■ May contain live or dead inactivated pathogen or
part of the pathogen.■ This provides a harmless first encounter with the
antigens of the potential pathogen
Benefits of Vaccination■ Provides immunity without suffering
the symptoms of the disease■ Gives long-term protection■ No long-term drug-taking required.
Examples of vaccines:MMR (Measles, mumps and rubella)3 in 1 (whooping cough, diptheria and tetanusBCG (tuberculosis)
HIGHER LEVEL
B-lymophocytes■ Produced and mature in the red
bone-marrow of the foetus■ Migrate to the lymphoid tissue■ If stimulated by antigen, a B-
lymphocyte cell multiplies and differentiates into huge numbers of:
Plasma cells – producing antibodiesMemory B cells – remain after infection
➢ B lymphocytes attack antigens in the blood or body fluids, by producing antibodies that surround the target.
➢ B lymphocytes move to the lymph nodes, where they acquire their receptor molecules.
B LYMPHOCYTES:
➢ Each B cell produces just one type of antibody.
➢ The plasma cells produce antibody molecules, that bind to pathogens and toxins that are circulating in tissues or body fluids, and mark them for destruction by monocytes.
➢ When a B cell encounters matching antigen, they divide into plasma and memory cells.
Memory B-cells■Give immediate protection
against future infections by the same pathogen or antigen
■ If they later detect a previous invader they rapidly reproduce
■This produces a large population of antibody-secreting plasma cells
➢ Memory B and T cells that form during the first response to an antigen, do not engage in that first battle.
➢ They circulate in the body for years, and can intercept antigens far quicker.
➢ This is how we achieve immunity against many diseases – either by getting the disease once or by preventing them using vaccines.
Long-term immunity
T-lymphocytes
■ Also known as T-cells■ Also produced in the foetal bone
marrow, but mature in the thymus■ Migrate to the lymphoid tissue■ Ignore free antigen - act against
virus-infected cells and cancerous cells
■ Multiply and differentiate rapidly when antigen binds to them
Helper T-cells■ Secrete chemicals which turn
the specific defence system “on” and “off”
■ The Helper T-cells– stimulate the production of B-cells– stimulate the formation of Killer T-
cells– accelerate the action of
phagocytes
Killer T-cells
■ Destroy – virus-infected cells– tumour cells– organ-transplant cells (“non-self”)
■ Secrete perforin (punches holes)■ Can stimulate target cell into
apoptosis (programmed cell death or “cell suicide”)
Destruction of Cancer Cell by a Killer T-Cell
Suppressor T-cells■ Maintain level of immune
response■ Inhibit
– B cells– T cells– Monocytes
■ Suppressor T’s stop the immune response when the infection has been defeated
Memory T-Cells• Like memory B-cells they survive a
long time and can respond to a specific invader in the futureMemory T-cells stimulate memory B-cells to start producing antibodies and they stimulate killer T-cells
Summary of T-cellsT-cells do not produce antibodies instead they act in one of four processes:
1. Helper T cells which recognise antigens, enlarge, andsecrete chemicals, such as interferon, which stimulate theproduction of B cells.
2. Killer T cells which attack cells containing a foreignantigen, secrete a chemical called perforin that perforatesthe membranes of cells
3. Suppressor T cells which stop immune responses
4. Memory T cells which can memorise the immunity, even
for life.
Summary of Lymphocytes
Lymphocyte interactions
■ B cell surface antibodies bind antigen
■ Helper T cells also bind, releasing interleukins
■ These chemicals stimulate B cells to multiply and differentiate
■ T cells encountering monocytes that are presenting antigen are stimulated to multiply and differentiate
Human Immunodeficiency Virus
Human Immunodeficiency Virus