The document provides an overview of the human immune system and how it responds to pathogens. It discusses the four main stages of the immune response: 1) phagocytosis where phagocytes engulf pathogens, 2) T-cell activation where T-cells recognize antigens, 3) B-cell activation and plasma cell production where B-cells secrete antibodies, and 4) antibody production where plasma cells secrete antibodies specific to the pathogen. It also covers primary versus secondary immune responses, vaccines, antigenic variation in pathogens, and the potential use of monoclonal antibodies to fight diseases like cancer.

1. IMMUNITY
THE IMMUNERESPONSE
If a pathogen getspastyourfirstline of defences,theimmunesystemkicks in
ACTIVATING THE IMMUNERESPONSE
 Whena pathogeninvade the body,the moleculesonitscell surface are identifiedasforeign,whichactivates
cellsinthe immune system
 The moleculesfoundonthe surface of the cellsare called antigens
 They`re usuallyproteinsorpolysaccharides
THE FOUR MAIN STAGES OF THE IMMUNE RESPONSE
1. PHAGOCYTOSIS
 A phagocyte isa type of white bloodcell thatcarriesout phagocytosis (engulfmentof pathogens
 They`re foundinthe bloodandin tissuesandare the firstcellsto respondtoa pathogeninside the body
 A phagocyte recognisesthe antigensonapathogen
 The cytoplasmof the phagocyte movesroundthe pathogen,engulfingit
 The pathogenisnowcontainedina phagocyticvacuole inthe cytoplasmof the phagocyte
 A lysosome thatcontainslysosomal enzymesfuseswiththe phagocyticvacuole
 The lysosomal enzymesbreakdownthe pathogen
 The phagocyte thenpresentsthe pathogen`santigens- itsticksthe antigensonit`ssurface to
activate otherimmune systemcells
2. T-CELL ACTIVATION
 A t-cell isa type of white bloodcell
 T-cellsare sometimescalledT-lymphocytes
 Lymphocytesare a type of WBC useful forprovidingimmunity
 T-cellsare made inthe marrow, butmature in the thymusgland
 T-cell hasproteinsonitssurface that bindto the antigenspresentedtoitbyphagocytes
 Thisactivatesthe T-cell
 Unlike B-cells,T-cellsdonotrecognise free-floatingantigens
 Rather,theirsurfacescontainspecializedantibody-like receptorsthatsee fragmentsof antigensonthe
surface of infectedorcancerouscells
 Differenttypesof T-cellsrespondindifferentways
HelperT-cells:
Coordinate immune responsesbycommunicatingwithothercells
Some stimulate nearb-cellstoproduce antibodies,otherscall inphagocytes
Cytotoxica.k.a. killerT-cells:
These performadifferentfunction
These cellsdirectlyattackothercellscarryingcertainforeignorabnormal moleculeson their
surfaces
Theyrelease PERFORIN tokill them
CytotoxicT-cellsare especiallyuseful forattackingvirusesbecause virusesoftenhidefromother
parts of the immune system,while theygrow inside infectedcells
CytotoxicT-cell recognise smallfragmentsof these virusespeekingoutfromthe cell membrane
and launchan attack to kill the infectedcell
3. B-CELL ACTIVATION AND PLAMA CELL PRODUCTION

2.  A type of WBC
 B-cellsare coveredwithantibodies-proteinsthatbindtoantigenstoforman antigen-antibodycomplex
 Each B-cell hasa differentshapedantibodyonitsmembrane,sodifferentonesbindtodifferentshaped
antigens
 Whenthe antibodyonthe surface of a B-cell meetsacomplementaryshapedantigen,itbindstoit
 This,togetherwithsubstancesreleasedfromT-cells,activatesthe B-cell
 The activatedB-cell thendividesbymitosistoproduce manymore B-cellscalledplasmacells- Allthe
cellsproducedwill be geneticallyidentical
 Thismeanstheyall produce identical antibodiesspecifictothe pathogen
4. ANTIBODY PRODUCTION
 The plasmacellsare identical tothe original B-cell (theyare clones)
 Plasmacellssecrete antibodiesdirectlyintothe bloodandthendie afterafew days
 These plasmacellsare notneededafterantibodiesare produced
 Antibodiessecretedare specifictothe pathogen
Antibodyfunctions include:
Coatingthe pathogentomake it easierfora phagocyte to engulf it
Coatingthe pathogentopreventitfromenteringhostcells
Bindingtoand neutralising(inactivating) toxinsproducedbythe pathogen
ANTIBODY STRUCTURE
 Antibodiesare proteins
 Theyare made up of chainsof aminoacidmonomerslinkedbypeptide bonds
 The specificityof anantibodydependsonitsvariable regions
 Each antibodyhasa differentshapedvariable region(due todifferentaminoacidsequences)that`s
complementarytoone specificantigen
 The constant regionsare the same inall antibodies
CELLULAR AND HUMORAL RESPONSES
The immune response isoftensplit into two:
 CELLULAR- The T-cellsformthe cellularresponse
 HUMORAL- B-cellsandthe productionof antibodiesformthe humoral response
PRIMARY AND SECONDARY IMMUNERESPONSES
PRIMARY RESPONSE
 Whenan antigenentersthe bodyforthe firsttime,itactivatesthe immune system
 Thisis calledthe primaryresponse
 The primary response isslowbecause there aren'tmanyB-cellsthatcanmake the antibodyneededtobind
to it
 Eventuallythe bodywillproduce enoughof the rightantibodytoovercome the infection
 Meanwhile the infectedpersonwillshow symptomsof the disease
 Afterbeingexposedtoanantigen,BothT-cell andB-cellsproduce memorycells
 These memorycellsremaininthe bodyforaslongtime
 MemoryT-cellsandmemoryB-cellsrememberthe specificantigenandwill binditasecondtime round
 The personis now immune- theirimmune systemhasthe abilitytorespondquicklytoa secondinfection

3. THE SECONDARY RESPONSE
 If the same pathogenentersthe bodyagain,the immune systemwillproduce aquicker,strongerimmune
response- the secondaryresponse
 MemoryB-cellsdivide intoplasmacellsthatproduce the rightantibodytothe antigen
 MemoryT-cellsdivide intothe correcttype of T-cellstokill the cell carryingthe antigen
 The secondaryresponse oftengetsridof the pathogenbefore youbegintoshow anysymptoms
VACCINES
 Vaccinescontainantigensthatcause yourbodyto produce memorycellsagainstaparticularpathogen,
withoutthe pathogencausingdisease
 Thismeansyou become immune withoutgettinganysymptoms
 Vaccinesprotectindividuals thathave them, and, because theyreduce the occurrence of the disease,those
not vaccinatedare alsolesslikelytocatchthe disease (there are fewerpeople tocatchit from)
 Thisis called herdimmunity
 Vaccinesalwayscontain-antigens-thesemaybe free orattachedto a dead or attenuated(weakened)
pathogen
 Vaccinesmaybe injectedortakenorally
 The disadvantages of takinga vaccine orallyare that itcouldbe brokendownbyenzymesinthe gutor the
moleculesof the vaccine maybe toolarge to be absorbedintothe blood
 Sometimesboostervaccinesare givenlaterontomake sure more memorycellsare produced
HOW A VACCINEWORKS?
 Vaccine containsantigensthatattachto a weakenordeadpathogen
 The phagocyte engulf thoughpathogen
 Cellspresentthe antigens- Thisactivates T-cells
 T-cellsdividebymitosistoproduce clones
 T-helpercellsreleasecytokines
 Cytokinesactivate B-cells
 B-cellsdividebymitosistoproduce plasmacells
 Plasmacellssecrete antibodies
 Antibodiesdestroypathogens
 Thisgivesa longerandstronger immune response
ETHICAL ISSUES SURROUNDING THE USE OF VACCINES
 All vaccinesare testedonanimalsbefore beingtestedonhumans
 Some people disagree withanimaltesting
 Also,animal basedsubstancesmaybe usedtoproduce a vaccine,whichsome peopledisagree with
 Testingvaccinesonhumanscan be riskye.g.volunteersmayputthemselvesatunnecessaryof contracting
the disease because theythinktheyare fullyprotected
 Some people don`twanttotake the vaccine due to the riskof side effects
 But theyare still protectedbecauseof herdimmunity
 Otherpeople thinkthisisunfair
 If there was an epidemicof a newdisease,therewouldbe rushtoreceive avaccine anddifficultdecisions
wouldhave tobe made aboutwhowouldbe the firstto receive it

4. ANTIGENICVARIATION
WHAT IS ANTIGENICVARIATION?
 Antigensonthe surface of pathogensactivate the primary response
 Whenyou're infectedasecondtime withthe same pathogen(whichhas the same antigensonits surface)
theyactivate the secondaryresponse andyoudon`tgetill
 However,some pathogenscanchange theirsurface antigens-pathogensof the same type thatshow
antigenicvariation,are oftenreferredtoasstrains
 Thisis calledantigenicvariation
 Differentantigensare formeddue tochangesinthe genesof a pathogen
 Thismeansthat whenyouare infectedforasecondtime,the memory cellsproducedfromthe first
infectionwill notrecognisethe differentantigens
 So the immune systemhasto start fromscratch andcarry out a primaryresponse againstthese new
antigens
 Thisprimaryresponse takestime togetridof the infection,whichiswhyyougetill again
 Antigenicvariationalsomakesitdifficulttodevelopvaccinesagainstsome pathogensforthe same reason
 Examplesof pathogensthatshow antigenicvariationinclude HIV andthe Influenzavirus
EXAMPLE: Antigenicvariation in the influenzavirus
 The influenzaviruscausesflu
 Proteinsonthe surface of the influenzavirusactas antigens,triggeringthe immunesystem
 These antigenscan change regularly,formingnew strainsof the virus
 Memorycellsproducedfrominfectionwithone strainof lfuwill notrecogniseotherstrainswithdifferent
antigens
 Thismeansyour immune systemproducesaprimaryresponse everytimeyouare infectedwithanewstrain
(carryingdifferentantigens)
 So thismeansyoucan sufferfromflumore than one-eachtime youare infectedwithanew strain
MONOCLONALANTIBODIES
THE USE OF MONOCLONALANTIBODIES
 Monoclonal antibodiesare antibodiesproducedformasingle groupof geneticallyidentical B-cells(plasma
cells)- Basicallycollectedfromone ancestral B-cell
 Thismeansthat theyare all identical instructure
 You can make monoclonal antibodiesthatbindtoanythingyouwante.g.a cell antigenorothersubstance,
and theywill onlybindto(target) thismolecule
FIGHTING CANCER
 Cancer cellshave differentantigenscomparedtonormal cells
 Monoclonal antibodies thatcomplementthese antigenswillbe harvestedandhave acytotoxicdrug
attachedto it
 However,there have beenissueswiththe actionof the antibodiesandthishasnotbeenassuccessful as
previouslyhoped