Si el cuerpo no tuviera defensas contra la infección, rápidamente sería colonizado por microorganismos. Estas defensas requieren un cuerpo vivo y que funcione correctamente. Un cadáver comienza a descomponerse casi de inmediato porque sus defensas ya no funcionan. Las barreras naturales son la piel, las membranas mucosas, las lágrimas, la cera de los oídos, el moco y el ácido del estómago. Además, el flujo normal de orina elimina los microorganismos que ascienden por el tracto urinario. El sistema inmunitario identifica y elimina los microorganismos que han atravesado las barreras naturales.

1. Pathogens and Our Defenses

2. ANTIBIOTICS
• How do antibiotics
work?
– Antibiotics take advantage
of cellular and molecular
differences between
prokaryotes (i.e. bacteria)
and eukaryotes (i.e. us).
– For example, penicillin
prevents Gram positive
bacteria from making their
cell wall.
• Antibiotics have no effect against viruses
or other non-bacterial pathogens.
• Overuse has caused many antibiotic-
resistant strains of bacteria to emerge.
Image Credit: Gerard D Wright.

3. VECTOR CONTROL
• How does vector control work?
– Vector control refers to any measures that reduce the population of
insect vectors carrying a particular pathogen
– Examples include insecticides, bed nets (mosquitos), reducing standing
water (mosquitos), cutting tall grass (ticks), and introducing predators
or insect pathogens
Cockroach infected with Metarhizium anisopliae
fungus, which is used in tick control
Image Credit: Chengshu Wang and Yuxian Xia.

4. FOOD SAFETY
• How does food safety work?
– Food safety refers to any measures that reduce the likelihood of
ingesting food-borne pathogens that are still infectious
– Examples include properly cooking meats and seafood, using separate
knives for meat and vegetables, and refrigerating food promptly
Image Credit: Bev Sykes

5. HAND WASHING
• How does hand washing work?
– Contrary to popular belief, soap does not kill pathogens unless it has
an antibacterial ingredient
– Soap helps dislodge or solubilize pathogens so that they can be
washes away by water
• To be effective, you should wash your hands with
soap and warm water for at least 20 seconds after
using the bathroom, changing diapers, and before
eating.
Image Credit: Wikimedia author Graf Foto

6. SKIN BARRIER
• How does our skin work?
– Skin cells have structures known as tight junctions between them that
act as a physical barrier to pathogens
– Microorganisms that are a normal part of our skin microbiome help
protect us from pathogens
– The outer layer of our skin is made of dead cells that regularly slough
off, taking any pathogens with them
Image Credit: S. Tsukita, et
al. Multifunctional strands
in tight junctions.
Nature Reviews Molecular
Cell Biology 2, 285-293
(April 2001)
doi:10.1038/35067088

7. MUCUS
• How does mucus work?
– Body cavities, such as your mouth or nose, are protected by mucous
membranes rather than skin
– Mucus is viscous and literally traps pathogens
– Mucus lies on top of cells with hair-like projections called cilia, which
also help trap pathogens
– Mucus also contains antimicrobial proteins like lysozyme
Image Credit: Rob Lee’s lab at University of Pennsylvania

8. TYPE I INTERFERONS
• How do interferons work?
– Type I interferons are a class of proteins secreted by cells that have
been invaded by a virus
– The interferons bind to receptors on neighboring cells and cause them
to make proteins that interfere with the ability of a virus to copy itself,
thereby limiting infection
– One of the type I interferons is used to treat hepatitis B and C
Image Credit: The Benjamin/Cummings Publishing Company, Inc.

9. NATURAL KILLER CELL
• How do natural killer cells work?
– All of our cells have a specific set of proteins on their surface that
identifies them as ‘self’ (major histocompatibility complex class I)
– Natural killer cells recognize cells that have too few of the ‘self’
proteins on their surface and therefore may be infected
– When an infected cells is recognized, the natural killer cell releases a
mixture of proteins that enter the infected cell and lead to cell death
(apoptosis)
Image Credit:
sphweb.bumc.bu.edu

10. • How do antibodies work?
– Antibodies are proteins on the surface of B cells
– Each B cell produces a different antibody variant
– If a B cell produces an antibody that binds strongly to a piece of a
pathogen (antigen), the B cell replicates itself and starts secreting
massive amounts of the antibody
– That circulating antibody binds to the pathogen or infected cell and
inactivates it
ANTIBODIES
Image Credit: Wikimedia author Je at uwo

11. KILLER T CELL
• How do killer T cells work?
– T cells have T cell receptor proteins on their surface and each T cell
produces a different T cell receptor variant, like B cells and antibodies
– T cell receptors recognize a piece of a pathogen (antigen) that is
“presented” by the ‘self’ proteins on the surface of an infected cell
– If a T cell receptor variant binds strongly, the T cell replicates itself and
also releases a mixture of proteins that enter the infected cell and lead
to cell death (apoptosis)
Image Credit: OpenStax College

12. COMPLEMENT
Courtesy of Kerry Laing, Fred Hutchinson Cancer Research Centre, Seattle, USA
• How does complement work?
– Complement proteins bind to Gram negative bacterial cells that live
outside of host cells and form a pore in the cell membrane called the
membrane attack complex
– The pore causes the cell to burst from osmotic pressure
• Complement cannot directly kill Gram positive
bacteria because their thick cell walls prevent
formation of the MAC

13. MACROPHAGE
Courtesy of Wikimedia author Mango Slices
• How do macrophages work?
– Macrophages have a set of pattern recognition receptors on their
surface that recognize common pathogen components, like sugars in
bacterial cell walls or lipids in parasite cell membranes
– When a receptor recognizes a pathogen, the macrophage engulfs it
(phagocytosis) and kills it by releasing digestive proteins into the
compartment

14. IRL IS MUCH MORE
COMPLICATED…