HUMAN BIOLOGY

sUmmary

section 9.1 The body protects itself

from pathogens with general and specific

responses of white blood cells and

chemicals they release. Inborn responses

provide innate immunity. Responses

after the body detects antigens of specific

pathogens provide adaptive immunity

(Table 9.5). An antigen is a molecule that triggers an immune

response against itself. Chemicals called cytokines help

organize or strengthen immune responses.

ExpLorEon yoUr oWn

174 Chapter 9

section 9.2 T and B cells are stationed

in lymph nodes, the spleen, and other parts

of the lymphatic system. Lymph vessels also

recover water and dissolved substances that

have escaped from the bloodstream and

return them to the general circulation.

section 9.3 The body’s first line of

defense against pathogens includes physical

barriers such as intact skin and mucous

membranes and chemical barriers such as

tears, saliva, gastric juice, and lysozyme

in mucus. Urine and diarrhea help flush

pathogens from the urinary tract and GI tract.

section 9.4 General, innate immune

responses may stop an infection from setting

in. Macrophages are “first responders”

that engulf and digest foreign agents and

clean up damaged tissue. Complement

proteins bind to pathogens and kill them by

inserting membrane attack complexes into

the invader’s plasma membrane. They also

attract phagocytes.

Activated complement and cytokines from macro phages

trigger inflammation, a fast, local response to tissue damage

(Table 9.6). Signs of inflammation include redness, warmth,

and pain. Chemical signals triggered by infection can cause

a fever.

The photograph in Figure 9.23 shows a reaction to a skin test for

tuberculosis. For this test, a health care worker scratches a bit of TB antigen

into a small patch of a patient’s skin. In people who have a positive reaction to

the test, a red swelling develops at the scratch site, usually within a day or two.

Even in a person with no medical history of the disease, this response is visible

evidence of immunological memory. It shows that there has been an immune

response against the tuberculosis bacterium, which the person’s immune system

must have encountered at some time in the past. Tests for allergies work the

same way.

In many communities, a TB test is required for people who are applying for jobs

that involve public contact, such as teaching in the public schools. To learn more

about this public health measure, find out if the test is required in your community,

where it is available, and why public health authorities believe it is important.

Dr. P. Marazzi/Science Source

Figure 9.23 This skin eruption indicates a

positive reaction to a tuberculosis skin test.

Nonspecific

Barriers

Intact skin; mucous membranes at other body surfaces

Infection-fighting chemicals in tears, saliva, gastric fluid

Resident bacteria that outcompete pathogens

Flushing effect of tears, saliva, urination, diarrhea, sneezing,

and coughing

Innate Immune Responses

Complement system

Inflammation

Fast-acting white blood cells (neutrophils, eosinophils,

and basophils), macrophages

Blood-clotting proteins, infection-fighting cytokines, NK cells

specific

Adaptive Immune Responses

T cells, B cells/plasma cells

Cytokines such as interleukins, other chemical weapons

(such as antibodies, perforins)

Table 9.5 The body’s Three Lines of Defense

Complement Proteins directly kill cells; stimulate lymphocytes

Cytokines Communication chemicals

Interleukins Cause inflammation and fever; cause

T cells and B cells to divide and specialize;

stimulate bone marrow stem cells; attract

phagocytes; activate NK cells

Interferons Confer resistance to viruses; activate NK cells

Table 9.6 some Chemical Weapons of immunity

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