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Innate defenses against infection include the skin, mucous membranes, and phagocytic cells
• Innate immunity is the body’s first line of defense against all invaders– Skin provides tough barrier and
general chemical defenses– Mucous membranes– Stomach acid– Hairs, cilia
• Pathogens that get past the body's external defenses are met by innate defensive cells– Found in blood and interstitial fluid– Macrophages are large phagocytic
cells– Natural killer cells release
chemicals– Specific proteins attack microbes
or impede their reproduction• Interferons help cells resist
viruses• The complement system– Initiated by microbes, can lead to
lysis of invaders
Bacteria
Co
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SE
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Virus Viral nucleic acid
New virusesInterferongenesturnedon
DNA
mRNA
Interferonmolecules
Host cell 1Makes interferon;is killed by virus
Host cell 2Protected against virusby interferon from cell 1
Interferon stimulatescell to turn on genesfor antiviral proteins
Antiviral proteins blockviral reproduction
The inflammatory response mobilizes nonspecific defense forces
• Tissue damage triggers the inflammatory response– Can disinfect tissues and limit further
infection• Steps of the inflammatory response
1. Tissue injury releases chemical signals such as histamine
2. Local blood vessels dilate and leakiness increases; phagocytes migrate to the area
3. Phagocytes consume bacteria and cell debris; tissue heals
• The inflammatory response may be widespread as well as localized– White blood cells may increase– Fever may stimulate
– The selected B cells multiply into clones of effector and memory cells
• Effector (plasma) cells* – Combat the antigen– Secrete antibody molecules that
circulate in blood and contribute to humoral immunity
– Last only 4 or 5 days• Memory cells – Remain in lymph nodes – May last for decades, sometimes
confer lifetime immunity
• Secondary immune response*– Memory cells exposed to same antigen a
second time– Second round of clonal selection ensues– Secondary response is faster and
stronger; produces very high levels of antibodies
Animation: Role of B Cells
Comparison of primary and secondary immune response Primary response
Takes several days to occur, during which the individual may become ill
Antibody level peaks in about two weeks, activated cells die out
Secondary response Occurs quickly Is of greater magnitude and lasts
longer
• Acquired immunity is specific: the body's response to a second antigen is not influenced by its response to the first one
Second exposureto antigen X,
first exposureto antigen Y
Secondary immuneresponse to
antigen X
Primary immuneresponse to
antigen X
Primary immuneresponse to
antigen Y
First exposureto antigen X
Antibodiesto X
Antibodiesto Y
An
tib
od
y c
on
ce
ntr
ati
on
0 7 14 21 28 35 42 49 56Time (days)
Antibodies are the weapons of humoral immunity
• Antibody molecules are secreted by plasma (effector) B cells
• Antibody molecule structure
– Y shaped, made of two identical "heavy" and two identical "light" polypeptide chains
– A C (constant) and a V (variable) region on each chain
– Antigen-binding sites specific to the antigenic determinants that elicited its secretion
• Antibody functions in humoral immunity
– Binds its antigen at the antigen-binding site
– Assists in elimination of the antigen, at the C region of the heavy chains
Antibodies mark antigens for elimination• Involves a specific recognition-and-attack
phase followed by a nonspecific destruction phase
• Antibodies mark invaders by forming antigen-antibody complexes
• Binding triggers ways to eliminate the invader– Neutralization – Agglutination of microbes– Precipitation of dissolved antigens – Activation of complement system
Binding of antibodies to antigensinactivates antigens by