Immune System Overview

Why an immune system?We are outnumbered! Viruses and bacteria

are everywhere!Humans offer limitless resources for

pathogens EnergyReproductive potential

Getting into the body isn’t easy!

Meet the enemyBacteria

Free-livingNot all are bad!Pathogenic ones produce toxins that damage

human tissueViruses

Obligate parasitesHijack human cells; convert to virus-producers,

killing host cell in the process(And fungi, protozoa too…)

A human fortressSkin is thick – hard to penetrateProduces substances that deter invasion:

Skin pH (not favorable)Mucus (sticky trap)Lysozymes (digest bacteria)

Specialized traps around vulnerable areas (Eyes, noes, mouth)Cilia sweep away invaders that are trappedStomach acid kills ingested invaders

…but we do get sick!Enter through weak points:

FoodNoseBreak in skin/scrapes

Cells are damaged/destroyed Dying cells release distress chemicals

(histamine) Triggers inflammation (blood vessel dilation,

increased blood flow) Draws defensive cells to area (generalized white

blood cells)

How do we tell “friend” from “foe”?All cells present antigens – surface

molecules that identify identity(antigen = antibody generator)

Immune system reacts to foreign antigens

A complex system!Several “lines” of defense:1. Barriers (First line of defense)2. Generalized defenders (Second line of

defense)3. Specific defenders AND memory (Third line of

defense)

Consist of:Several types of cellsProteins

The Complement SystemPart of second line of defenseFree-flowing proteins found in bloodQuickly reach site of invasionReact to antigensWhen activated, can

Trigger inflammationAttract “eater cells” (macrophages)Coat pathogen (make macrophages’ job easier)Kill intruder directly

PhagocytesFind and “eat” bacteria, viruses, dead/injured

body cells by phagocytosis3 types:

Granulocytes (some)MacrophagesDendritic cells

GranulocytesOften first to site of infectionNumerousShort lifespan“Pus” in infected wounds chiefly composed of

granulocytes

Macrophage“Big eaters”Slower to respond to invader than

granulocyteLarger, longer-lived, more capable

Help alert rest of immune system to invaderStart as monocytes; become macrophages

when entering bloodstream

Dendritic cells“Eater” cells Help with immune system activation – act as

antigen-presenting cellsFilter bodily fluids to clear foreign organisms

and particles

3rd Line of Defense: Humoral/Fluids Cell-MediatedFocus: catch invader

before infection of cellsIncludes Antibodies to

deactivate/eliminate threat in bodily fluids

Deals with infection that has already entered cells

Focus: destruction of infected cells so it doesn’t spread!

Lymphocytes – Part of both humoral and cell-mediatedT and B cellsOriginate in bone marrowMigrate to lymph nodes, spleen, thymus to

matureLymph vessels

transport, store lymphocytesFeeds cells into bodyFilter out dead cells/invading organisms

ReceptorsEach lymphatic cell contains surface

receptors Recognize foreign antigens as cells pass by…Specialized for a particular antigen

T cells – Cell-mediated immunityPrimary focus = infected cellsTwo types: helper and killerT = thymus (site of maturation)

Helper T cellMain regulator of third line of defensePrimary task: activate other cells (B and

killer T cells)Usually activated by macrophages/dendritic

cells (during antigen presentation)

Killer T cellDirectly attacks body cells infected by

pathogen, cancer cellsReceptors used to determine if each cell

encountered is self/non-self (compare to accepted receptors, MHC)

B lymphocyte cell – Humoral immunitySearches for intact antigens matching

receptorIf a match is found…

Connects to antigenTriggering signal set off…

T helper proteins help fully activate B cellProduces 1000’s of clones: differentiate into

plasma cells or B memory cells

Plasma CellProduces antibodiesResponds to same antigen matched by B cell

receptorSeek out intruders, help destroy themRelease tens of thousands/second

AntibodiesY-shapedAttach to matching antigens

Enhance phagocytosis of macrophages (label for capture)

Neutralize toxinsIncapacitate viruses (coat surface proteins)Group pathogens by linking (agglutination)

5 types of antibodies (Ig’s):IgG Most common; fight bacterial infections; pass from mom to

child in placenta (G = mom’s gift)

IgA In mucous membranes of digestive system, milk, tears, saliva (A = a lot of mucus)

IgM Natural defenses against general bacterial infections (M=most bacteria)

IgE Stimulate basophils and mast cells to defend against parasites fungi and worms (E=eeeww!)

IgD On membranes of B-lymphocytes; form plasma and memory cells (D=defend blood)

Memory cellsProlonged lifespan“Remember” specific intrudersBoth B and T cells have memory cellsHelps trigger immune system to respond

more quickly if invader reappears

InflammationOutcome of secondary immune responseIncreases blood flow to affected area

Blood vessels dilate to increase blood flowImmune cells go to affected areaImmune response takes place at the site it’s neededTissues = red and swollen because of the blood that

enters the area; increasing temperature = antimicrobial

Pain from pressure of swollen tissues on nerve endingsNormal functions return when the tissue is fully

recovered

Immunization and VaccinationNatural Immunity

Natural: exposed to foreign antigens as part of everyday life Acquired immunity – body responds to foreign

antigens and develops immunity using B and T lymphocytes

Passive immunity Embryological development when antibodies

(Ig’s) from the mother’s blood stream are passed to the fetus

Post-natal – baby receives antibodies via milk

Artificial ImmunityActive: Vaccinations

Therapeutic exposure to antigensStimulates the secondary response by

introducing pathogenic material (inactivated, attenuated, or partial) into the body

Vaccines are typically used for viruses! Antibiotics are only for bacteria

Passive: Antibody transferPatient receives (via injection) large amounts

of antibodies to fight disease Globulin injections can remove certain

microorganisms from the body