The Immune System

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Pathogens have evolved mechanisms to attack immune responses.

Through antigenic variation, some pathogens are able to change epitope expression and prevent recognition.

The human influenza virus mutates rapidly, and new flu vaccines must be made each year.

Human viruses occasionally exchange genes with the viruses of domesticated animals.

This poses a danger as human immune systems are unable to recognize the new viral strain.

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Latency

Some viruses may remain in a host in an inactive state called latency.

Herpes simplex viruses can be present in a human host without causing symptoms.

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Attack on the Immune System: HIV

Human immunodeficiency virus (HIV) infects helper T cells.

The loss of helper T cells impairs both the humoral and cell-mediated immune responses and leads to AIDS.

HIV eludes the immune system because of antigenic variation and an ability to remain latent while integrated into host DNA.

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The progress of an untreated HIV infection

Latency

Relative antibody

concentration

AIDS

Helper T cell concentration

in blood (cells/mm3)

Helper T cell

concentration

Relative HIV

concentration

Years after untreated infection

0

1

2

3

4

5

6

7

8

9

10

0

200

400

600

800

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People with AIDS are highly susceptible to opportunistic infections and cancers that take advantage of an immune system in collapse.

The spread of HIV is a worldwide problem.

The best approach for slowing this spread is education about practices that transmit the virus.

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Cancer and Immunity

The frequency of certain cancers increases when the immune response is impaired.

Two suggested explanations are

Immune system normally suppresses cancerous cells

Increased inflammation increases the risk of cancer

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Review

Stem cell

Cell division and gene rearrangement

Antigen

Clonal selection

Elimination of

self-reactive

B cells

Formation of activated cell populations

Antibody

Microbe

Memory cells

Effector B cells

Receptors bind to antigens

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You should now be able to:

Distinguish between innate and acquired immunity.

Name and describe four types of phagocytic cells.