How the Animal Body Defends Itself

Three Lines of Defense - the skin, roaming cells that attack and engulf pathogens, and the highly specific immune system.

Skin: The First Line of Defense  (Fig. 33.1)

The skin is the largest organ of the body and is composed of three layers. (Fig. 33.2)

outer epidermis


subcutaneous layer

Other External Surfaces

sticky mucus and cilia

Cellular Counterattack: The Second Line of Defense

 The Lymphatic System (
Fig. 33.3)

Cells That Kill Invading Microbes: all white blood cells

Macrophages (Fig. 33.4)  Phagocytosis Movie


Natural killer cells (
Fig. 33.5) Killing a cancer cell

Proteins That Kill Invading Microbes

The complement system is made up of a series of inactive plasma proteins that become activated when they encounter fungi and bacteria in the body.

Complement proteins (
Fig. 33.6)

interferon  Movie

Gamma interferon

The Inflammatory Response (Fig. 33.7)

chemical alarm signal




The Temperature Response


Specific Immunity: The Third Line of Defense

Cells of the immune system (Table 33.1a) and (Table 33.1b).

Leukocytes (white blood cells) include macrophages, neutrophils and natural killer cells


MHC proteins


 B cells and T cells are also called lymphocytes

T cells:

helper T cells: initiate the response

memory T cells: provide a quick response to infection

cytotoxic T cells: lyse infected cells

suppressor T cells: stop the immune response

T cells can only bind to antigens on the surface of cells.

B cells

plasma cells

memory B cells.


Initiating the Immune Response



Antigen-presenting cells (Fig. 33.8)


interleukin-1 activates the helper T cells, which in turn activate both T cells and B cells, by releasing interleukin-2.

T Cells: The Cellular Response (Fig. 33.9)

Interleukin-2 mobilizes a type of T cell known as a cytotoxic T cell.


special receptor protein

clone of cells

B Cells: The Humoral Response (Fig. 33.10) Movie

Interleukin-2 also activates B cells to attack bacteria

surface receptors

receptor proteins called antibodies. (
Fig. 33.11)

clone of B cells


plasma cells


memory B cells

Summary Movie of the specific immune system response.

The vertebrate immune system is able to recognize virtually every foreign molecule with which it comes in contact.

Active Immunity Through Clonal Selection

clonal selection.

primary immune response (
Fig. 33.13)

secondary immune response

Summary of immune response (Fig. 33.14)

Vaccination (Movie)

vaccination (Fig. 33.15)

dead or disabled pathogen

"piggyback" vaccines (Fig. 14.11)

Flu. Flu epidemic of 1918 (Fig. 33.16). 20 million Americans and Europeans died in 18 months.

HIV vaccine (Fig. 33.17)

Immune System Failure

Autoimmune diseases represent failures of the immune system. Sometimes the immune system begins to attack specific cells in the body, as it does in diabetes, rheumatoid arthritis, and multiple sclerosis.

Allergies. (
Fig. 33.20)

mast cells release histamines that trigger mucus production and nasal congestion. (Movie)



Common allergens include feces of dust mites (
Fig. 33.19), pollen, foods, fur, and other by-products of living organisms.


AIDS: Immune System Collapse

AIDS (Acquired Immune Deficiency Syndrome) is caused by HIV


 (human immunodeficiency virus).

This was first recognized as a disease in 1981 and infected 46 million people worldwide by the year 2002. The AIDS epidemic in the U.S. (Fig. 33.22).




T4 cell survival (Fig. 33.21).


HIV life cycle

Opportunistic infections:
development of HIV untreated

Contracting HIV: blood, semen, and vaginal secretions have large quantities of macrophages, so HIV is transmitted primarily through sharing intravenous drug needles or through sexual contact.

Tears, saliva, and urine have very low levels of macrophages, so transmission of HIV through casual touching, kissing, or using toilets is highly unlikely.

protease inhibitors, AZT


To reduce chances of contracting AIDS:

Don't reuse needles

Abstain from sex.

Have a monogamous relationship with a partner known to be free of HIV.

Use latex or polyurethane condoms.