Chapter 40 Immunity

I. Nonspecific Defense Responses
A. Barriers to Invasion [of pathogens]
1 Intact skin is an important barrier.
2 Ciliated, mucous membranes in the respiratory tract sweep out bacteria and particles.
3 Exocrine glands secrete lysozymes, which degrade the bacterial cell wall.
4 Gastric fluid in the stomach kills many pathogens.
5 The normal microbial inhabitants of the skin, gut, and vagina keep the growth of pathogens in
check.
B. Phagocytes: The Macrophages and Their Kin
1 White blood cells (neutrophils, eosinophils, monocytes = macrophages) engulf and destroy
invading agents.
2 Phagocytes are distributed in blood, tissue spaces, lymph nodes, spleen, and other organs.
C. Complement System
1 The complement system is a set of plasma proteins that enhance nonspecific and specific defense
responses.
2 These proteins are activated in a cascading fashion when barriers to fungal and microbial
infection are breached; the results are as follows:
a. Chemical gradients of proteins attract phagocytes to the scene.
b. Complement proteins coat the surface of invading cells—attracting phagocytes.
c. Other proteins cause the pathogen to lyse and die.
D. Inflammation
1 While complement proteins are being activated, basophils and mast cells secrete histamine, which
promotes leakage of fluid out of capillaries.
2 Inflammatory response results include:
a. Localized warming and redness occur at the site of damage or invasion.
b. Fluid seeps from blood vessels causing swelling and delivery of infection-fighting proteins to
the tissues.
c. Phagocytes engulf foreign invaders and debris.
d. Clotting mechanisms help wall off the pathogen and promote repair of tissues.

II. Specific Defense Responses: The Immune System
A. Introduction
1 Phagocytes summoned to an inflammation site are indiscriminate engulfers and may not be enough
to check the spread of an invader.
2 Macrophages and T and B lymphocytes of the vertebrate immune system may be needed.
3 Interactions among these cells are the basis of the vertebrate immune system.
4 This system shows specificity and memory.
B. The Defenders: An Overview
1 The names and functions of white blood cells responsible for immune responses are as follows: a. Macrophages are phagocytic and alert helper T cells to the presence of specific foreign
agents.
b. B cells and their progeny (plasma cells) produce antibodies, which are specific substances
that tag targets for destruction.
c. Cytotoxic T cells destroy body cells already infected with viruses or parasitic fungi.
d. Helper T cells stimulate division of B cells and cytotoxic T cells.
e. Suppressor T cells slow down or prevent immune responses.
f. Memory cells (derived from T or B cells) quickly respond to subsequent invasions.
2 T cells carry out “cell-mediated” response; B cells carry out “antibody-mediated” response. C. Recognition of Self and Nonself
1 Your own cells (“self”) bear surface proteins (MHC markers), which your lymphocytes ignore. 2 But “nonself” cells (viruses, bacteria, bee venom, organ transplants) bear antigens that trigger
immune responses.
D. Primary Immune Response
1 Antibody-Mediated Immune Response
a. B lymphocytes (and plasma cells derived from them) bear antibodies (Y-shaped) on their
surface.
b. When bacteria enter the body, macrophages engulf them and eventually display the bacterial
antigens along with their own MHC markers on the cell surface.
c. When a “virgin” B cell makes contact with other bacteria that escaped the macrophages, it
will bind to the bacterial antigen and become sensitive to communication signals from
macrophages and helper T cells.
d. Helper T cells lock onto the antigen-MHC complexes on the macrophage surface causing the
macrophages to release an interleukin that stimulates the helper T cells to release their
own interleukins.
e. In the presence of interleukins, B cells sensitized to the antigen will divide rapidly to produce
clone cells, all making the same antibody; part of the clone population differentiates into
plasma cells that continue to make antibody.
f. Antibodies (immunoglobulins) do not destroy pathogens but rather mark them for disposal by
other means.
1) IgM and IgG enlist the aid of macrophages and complement proteins.
2) IgA, present in saliva, tears, and mucus, helps repel invaders at the start of the
respiratory system.
3) The IgE antibodies stimulate mast cells to secrete histamine.
4) IgD and IgM work together to help bind antigen to B cells.
g. Targets of antibodies include bacteria and extracellular phases of viruses, fungi, and
protozoa.
2 Cell-Mediated Immune Response
a. Cytotoxic T cells destroy body cells that are already infected (mainly with viruses).
1) The forerunners of cytotoxic T cells arise from stem cells in the bone marrow from
whence they travel to the thymus gland to become mature.
2) While circulating in the blood they can recognize the antigen-MHC complexes on the
infected cells, the killer Ts punch holes with a chemical (perforin) and destroy the
cells before they can multiply.
3) Cytotoxic T cells also attack organ transplants as well as mutant and cancerous cells.
b. Natural killer (NK) cells kill tumor cells and virus-infected cells spontaneously, without the
presence of antibodies.
3 Control of Immune Responses
a. Both types of immune responses are regulated events.
b. Feedback inhibition and suppressor T cells reduce the intensity of attack.
E. Antibody Diversity and the Clonal Selection Theory
1 All B cells have the same genes coding for the polypeptides in each arm of the antibody molecule,
but different polypeptides can be made by shuffling the genes into millions of combinations to
produce antibodies against numerous agents.
2 The clonal selection theory proposes that a lymphocyte activated by a specific antigen will divide
and give rise to a clone of cells that are specific only to that antigen.
F. Secondary Immune Response
1 “Immunological memory” is the basis of the secondary immune response to a previously
encountered agent.
2 It is more rapid, of greater magnitude, and of longer duration.
3 After a primary immune response, some B and T cells continue to circulate for years as memory
cells, which can divide when they meet the antigen again.

III. Immunization
A. Immunization involves a deliberate production of memory cells by a vaccine that is made from killed
or weakened bacteria or viruses.
B. It is also possible to incorporate antigen-encoding genes from one pathogen into a different organism. C. If a person has already been exposed to bacterial pathogens, passive immunity can be temporarily
conferred by injecting antibodies.

IV. Abnormal or Deficient Immune Responses
A. Allergies
1 An allergy is a secondary immune response to a normally harmless substance.
2 Exposure triggers production of IgE antibodies, which cause the release of histamines and
prostaglandins.
3 A local inflammatory response results; death can even occur due to anaphylactic shock, a
condition in which air passages leading to the lungs constrict, fluid escapes too rapidly from
capillaries, and blood pressure drops.
B. Autoimmune Disorders
1 In autoimmune disorders, lymphocytes turn against the body’s own cells.
2 Rheumatoid arthritis is an inflammation of the joints caused by antibody that treats the body’s
own IgG molecules as if they were antigens.
C. Deficient Immune Responses
1 When cell-mediated immunity is weakened, infections that would normally not be serious become
life-threatening.
2 In acquired immune deficiency syndrome (AIDS), the cause is the human immunodeficiency virus
(HIV).
D. Case Study: The Silent Unseen Struggles
1 Suppose you step on a tack and several thousand bacteria are brought into your body.
2 Blood pools and clots around the wound.
3 Basophils and mast cells secrete histamine; vasodilation and greater permeability increase the
supply of the complement system.
4 Phagocytes engulf some foreign objects and bacteria, but the bacterial divisions outpace the
phagocytes.
5 Some memory lymphocytes join with antigens and become activated—they divide rapidly in
response.
6 By the third day, a peak of antibody production turns the tide and continues for two weeks until
all invaders are killed.
7 Memory lymphocytes continue to circulate and wait for a future battle.