Why an immune system?
 We are outnumbered! Viruses and bacteria are
everywhere!
 Humans offer limitless resources for pathogens
 Energy
 Reproductive potential
 Getting into the body isn’t easy!
Meet the enemy
 Bacteria
 Free-living
 Not all are bad!
 Pathogenic ones produce toxins that damage human
tissue
 Viruses
 Obligate parasites
 Hijack human cells; convert to virus-producers, killing
host cell in the process
 (And fungi, protozoa too…)
A human fortress
 Skin is thick – hard to penetrate
 Produces 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
trapped
 Stomach acid kills ingested invaders
…but we do get sick!
 Enter through weak points:
 Food
 Nose
 Break 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 cells
 Proteins
The Complement System
 Part of second line of defense
 Free-flowing proteins found in blood
 Quickly reach site of invasion
 React to antigens
 When activated, can
 Trigger inflammation
 Attract “eater cells” (macrophages)
 Coat pathogen (make macrophages’ job easier)
 Kill intruder directly
Phagocytes
 Find and “eat” bacteria, viruses, dead/injured body
cells by phagocytosis
 3 types:
 Granulocytes (some)
 Macrophages
 Dendritic cells
Granulocytes
 Often first to site of infection
 Numerous
 Short lifespan
 “Pus” in infected wounds chiefly composed of
granulocytes
Macrophage
 “Big eaters”
 Slower to respond to invader than granulocyte
 Larger, longer-lived, more capable
 Help alert rest of immune system to invader
 Start as monocytes; become macrophages when
entering bloodstream
Dendritic cells
 “Eater” cells
 Help with immune system activation – act as antigen-
presenting cells
 Filter bodily fluids to clear foreign organisms and
particles
rd
3
Line of Defense:
Humoral/Fluids
Cell-Mediated
 Focus: catch invader before
 Deals with infection that has
infection of cells
 Includes Antibodies to
deactivate/eliminate threat in
bodily fluids
already entered cells
 Focus: destruction of infected
cells so it doesn’t spread!
Lymphocytes – Part of both
humoral and cell-mediated
 T and B cells
 Originate in bone marrow
 Migrate to lymph nodes, spleen, thymus to mature
 Lymph vessels
 transport, store lymphocytes
 Feeds cells into body
 Filter out dead cells/invading organisms
Receptors
 Each lymphatic cell contains surface receptors
 Recognize foreign antigens as cells pass by…
 Specialized for a particular antigen
T cells – Cell-mediated immunity
 Primary focus = infected cells
 Two types: helper and killer
 T = thymus (site of maturation)
Helper T cell
 Main regulator of third line of defense
 Primary task: activate other cells (B and killer T cells)
 Usually activated by macrophages/dendritic cells
(during antigen presentation)
Killer T cell
 Directly attacks body cells infected by pathogen,
cancer cells
 Receptors used to determine if each cell encountered
is self/non-self (compare to accepted receptors,
MHC)
B lymphocyte cell – Humoral
immunity
 Searches for intact antigens matching receptor
 If a match is found…
 Connects to antigen
 Triggering signal set off…

T helper proteins help fully activate B cell
 Produces 1000’s of clones: differentiate into plasma
cells or B memory cells
Plasma Cell
 Produces antibodies
 Responds to same antigen matched by B cell receptor
 Seek out intruders, help destroy them
 Release tens of thousands/second
Antibodies
 Y-shaped
 Attach to matching antigens
 Enhance phagocytosis of macrophages (label for capture)
 Neutralize toxins
 Incapacitate viruses (coat surface proteins)
 Group pathogens by linking (agglutination)
5 types of antibodies (Ig’s):
Most common; fight bacterial infections; pass from mom to
IgG child in placenta (G = mom’s gift)
In mucous membranes of digestive system, milk, tears, saliva
IgA (A = a lot of mucus)
Natural defenses against general bacterial infections (M=most
IgM bacteria)
Stimulate basophils and mast cells to defend against parasites
IgE  fungi and worms (E=eeeww!)
On membranes of B-lymphocytes; form plasma and memory
IgD cells (D=defend blood)
Memory cells
 Prolonged lifespan
 “Remember” specific intruders
 Both B and T cells have memory cells
 Helps trigger immune system to respond more quickly
if invader reappears
Inflammation
 Outcome of secondary immune response
 Increases blood flow to affected area
 Blood vessels dilate to increase blood flow
 Immune cells go to affected area
 Immune response takes place at the site it’s needed
 Tissues = red and swollen because of the blood that
enters the area; increasing temperature = antimicrobial
 Pain from pressure of swollen tissues on nerve endings
 Normal functions return when the tissue is fully
recovered
Immunization and Vaccination
 Natural 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 Immunity
 Active: Vaccinations
 Therapeutic exposure to antigens
 Stimulates 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 transfer
 Patient receives (via injection) large amounts of
antibodies to fight disease

Globulin injections can remove certain microorganisms from
the body