AP Biology Notes Outline
Chapter 43: The Body’s Defenses
An animal must defend itself from the many
dangerous pathogens it may encounter in the
environment.
Two major kinds of defense have evolved that
counter these threats:
 innate immunity;
 acquired immunity.
CONCEPT 43.1 – Innate immunity provides broad defenses against infection. It is present before any exposure
to pathogens and is effective from the time of birth. It involves nonspecific responses to pathogens. With both
external and internal innate immunity, one infectious agent is not distinguished from another.
The innate immune responses include barrier defenses as well as defenses to combat pathogens that enter the body.
1.
BARRIER DEFENSES – skin and mucous membranes
provide a physical barrier and also produce secretions that result
in a skin pH from 3 to 5, and the antimicrobial lysozyme found
in saliva, mucous secretions, and tears.
2.
CELLULAR INNATE DEFENSES combat pathogens that get
through the skin. They include phagocytic WBCs and
antimicrobial proteins:
a. Neutrophils: WBCs that ingest and destroy microbes
via phagocytosis.
b. Monocytes: WBCs that migrate into tissue and
develop into macrophages, giant phagocytic cells.
c. Eosinophils: WBCs that defend against parasitic
invaders such as worms by discharging hydrolytic
enzymes.
d. Interferon: an antimicrobial protein against viral
infections – causes cells adjacent to infected cells to
produce substances to inhibit viral replication.
e. Complement System: consists of ~30 proteins with a
variety of functions – including lysing invading cells.
3.
LOCAL INFLAMMATORY
RESPONSE – triggered by damage to
tissue or the entry of pathogens.
a. Leads to release of numerous
chemical signals such as
histamines, which are released
by basophils and mast cells
(both types of WBCs) in
response to injury.
b. Histamines trigger the dilation
and permeability of nearby
capillaries – this dilation aids
in delivering clotting agents
and phagocytic cells to the
injured area.
4. NATURAL KILLER CELLS – help to recognize and remove diseased cells. NKC’s often trigger apoptosis in the cells they
attack. Apoptosis is programmed cell death brought about by signals that trigger the activation of a cascade of “suicide”
proteins in the cells destined to die.
AP Biology Notes Outline
Chapter 43: The Body’s Defenses
FIGURE 43.4 - The Lymphatic System - plays an active role in defending the body from pathogens.
1) Interstitial fluid bathing the tissues, along with the white blood cells in it, continually enters lymphatic capillaries.
2) Fluid inside the lymphatic capillaries, called lymph, flows through lymphatic vessels throughout the body.
3) Within lymph nodes, microbes and foreign particles present in the circulating lymph encounter macro-phages, dendritic cells,
and lymphocytes, which carry out various defensive actions.
4) Lymphatic vessels return lymph to the blood via two large ducts that drain into veins near the shoulders
CONCEPT 43.2 – In acquired immunity, lymphocytes provide specific defenses against infection. Also called
adaptive immunity, it develops only after exposure to inducing agents such as microbes, toxins, or other foreign
substances. It involves a very specific response to pathogens.
In acquired immunity, lymphocyte receptors provide pathogen-specific
recognition. Vertebrates have two types of lymphocytes: B cells and T cells,
both of which arise from stem cells in the bone marrow and circulate through
the blood and lymph. Both recognize particular microbes:
1. B LYMPHOCYTES: proliferate in the bone marrow. B cells make
antibodies, which provide humoral immunity. This helps to fight
pathogens that are circulating in the blood.
2. T LYMPHOCYTES: mature in the thymus. Cytotoxic T cells destroy
body cells that are infected by a pathogen or cancer cells, and Helper
T cells activate both B and T cells during acquired immunity.
3. ANTIGENS: foreign molecules that elicit a response by lymphocytes.
B and T cells recognize them by specific receptors imbedded in their
plasma membranes. A lymphocyte actually recognizes and binds to
just a small, accessible portion of the antigen called an epitope.
4. ANTIBODIES: soluble proteins secreted by B cells during an immune response.
5. B or T CELL ACTIVATION: occurs when an antigen binds to a B or
T cell.
6. B-CELL ACTIVATION: is enhanced by cytokines. The lymphocyte
forms two clones of cells in a process called clonal selection. The
result is thousands of cells, all specific to this antigen.
a. Effector Cells: combat the antigen.
b. Memory Cells: are long-lived, bear receptors for the same
antigen, and can quickly mount an immune response in
subsequent infections.
7. ANTIGEN BINDING: B cell receptors bind intact antigens, and T cell receptors bind antigens that are displayed by antigenpresenting cells on the MHCs.
8. MHCs (MAJOR HISTOCOMPATIBILITY COMPLEX MOLECULES): proteins that are the product of a group of
genes – and all individuals differ in their MHCs. This is a major component of “self”. There are 2 types of MHCs:
a. Class I MHCs – found on almost all cells of the body, except RBCs.
b. Class II MHCs – made by dendritic cells, macrophages, and B cells.
9. B and T CELL SPECIFICITY: is a result of the shuffling and recombination of several gene segments and results in more
than 1 million different B cells and 10 million different T cells. Each B or T cell responds to only one antigen!
AP Biology Notes Outline
Chapter 43: The Body’s Defenses
FIGURE 43.15 – PRIMARY v. SECONDARY
IMMUNE RESPONSE
A primary immune response occurs when the body is first
exposed to an antigen and a lymphocyte is activated.
A secondary immune response occurs when the same
antigen is encountered at a later time. It is faster and of
greater magnitude.
CONCEPT 43.3 – Humoral and cell-mediated immunity defend against different types of threats. These are
both types of acquired immunity!
1.
2.
HUMORAL IMMUNITY: involves the activation and clonal selection of effector B cells, which produce antibodies that
circulate in the blood. Humoral immunity defends against extracellular pathogens by binding to antigens, thereby neutralizing
pathogens or making them better targets for phagocytes and complement proteins.
CELL-MEDIATED IMMUNITY: involves the activation and clonal selection of cytotoxic T cells, which identify and destroy
infected cells. Cell-mediated immunity defends against intracellular pathogens and cancer by binding to and lysing the infected
cells or cancer cells.
AP Biology Notes Outline
Chapter 43: The Body’s Defenses
Figure 43.7 – The Role of Helper T Cells in Humoral and Cell Mediated Immunity:
Helper T cells aid both humoral and cellular responses. When activated by interaction with the class II MHC molecule of an antigen
presenting cell, they secrete cytokines that stimulate and activate both B cells and cytotoxic T cells. The TH cell is bond to the class II
MHC by a CD4 protein.
Figure 43.18 – The Killing Action of Cytotoxic T Cells
Cytotoxic T cells bind to class I MHC molecules, displaying antigenic fragments on the surface of infected body cells. The T c is
bound to the infected cell’s class I MHC by its CD8 protein. T c cells destroy infected body cells.
AP Biology Notes Outline
Chapter 43: The Body’s Defenses
FIGURE 43.19 - B Cells: A Response to Extracellular Pathogens
Activation of B cells is aided by cytokines and antigen binding to helper T cells. The clonal selection of B cells generates antibodysecreting plasma cells, the effector cells of humoral immunity. B cells produce memory cells as well as plasma cells. The plasma
cells secrete antibodies in prodigious numbers – these will circulate in the blood, and bind to and destroy the antigen. The memory
cells are long-lived and will recognize the same antigen if the body is exposed a 2 nd time.
Clone of Memory B
Cells
FIGURE 43.21 – The Five Classes of Immunoglobulins
Modes of antibody action include: (1) neutralization – antibodies bind to pathogen’s surface proteins, which prevents it from entering
and infecting cells; (2) opsonization – results in increased phagocytosis of the antigen; and (3) lysis – caused by activation of the
complement system.
AP Biology Notes Outline
Chapter 43: The Body’s Defenses
Active & Passive Immunization
Active immunity develops naturally in response to an infection or it can develop following immunization, also called vaccination. In
immunization, a nonpathogenic form of a microbe or part of a microbe elicits an immune response to an immunological memory for
that microbe
Passive immunity, which provides immediate, short-term protection, is conferred naturally when IgG crosses the placenta from mother
to fetus or when IgA passes from mother to infant in breast milk. It can be conferred artificially by injecting antibodies into a
nonimmune person.
Tissue Transplant, Organ Donation, and Blood Typing
The immune system’s ability to distinguish self from nonself limits tissue transplantation. The immune system can wage war against
cells from other individuals. MHC molecules are responsible for stimulating the rejection of tissue grafts and organ transplants. the
chances of successful transplantation are increased if the donor’s tissue-bearing MHC molecules closely match the recipient’s. The
recipient must also take immunosuppressant drugs.
• Transplanted tissues are usually destroyed by the recipient’s immune system. MHC molecules are responsible for stimulating
the rejection of tissue grafts and organ transplants. The chances of successful transplantation are increased if the donor and
recipient MHC tissue types are well matched or if the recipient is given immunosuppressive drugs.
• Lymphocytes in bone marrow transplants may cause a graft versus host reaction in recipients
• Certain antigens on red blood cells determine whether a person has type A, B, AB, or O blood. Antibodies to nonself blood
types already exist in the body. Transfusion with incompatible blood leads to destruction of the transfused cells
Diseases of the Immune System
Exaggerated, self-directed, or diminished immune responses can cause disease:
1) Allergies & The Allergic Response:
Allergies are exaggerated (hypersensitive) responses to certain antigens called allergens
1. IgE antibodies produced in response to initial exposure to an allergen bind to receptors or mast cells.
2. On subsequent exposure to the same allergen, IgE molecules attached to a mast cell recognize and bind the allergen.
3. Degranulation of the cell, triggered by cross-linking of adjacent IgE molecules, releases histamine and other chemicals,
leading to allergy symptoms.
• An acute allergic response sometimes leads to anaphylactic shock. A whole-body, life-threatening reaction that can occur
within seconds of exposure to an allergen
2) Autoimmune Diseases:
In individuals with autoimmune diseases the immune system loses tolerance for self and turns against certain molecules of the body
• rhumatoid arthritis is an autoimmune disease that leads to damage and painful inflammation of the cartilage and bone of joints
•
lupus: the immune system generates antibodies (autoantibodies) against a wide range of self molecules, including histones and
DNA released by the normal breakdown of body cells.
•
insulin-dependent diabetes: the insulin-producing beta cells of the pancreas are the targets of autoimmune cytotoxic T cells
•
multiple sclerosis: T cells infiltrate the central nervous system and destroy the myelin sheath that surrounds some neurons
•
An inborn or primary immunodeficiency results from hereditary or congenital defects that prevent proper functioning of innate,
humoral, and/or cell-mediated defenses.
•
An acquired or secondary immunodeficiency results from exposure to various chemical and biological agents. In severe
combined immunodeficiency (SCID) both the humoral and cell-mediated branches of acquired immunity fail to function.
•
Acquired Immunodeficiency Syndrome (AIDS): People with AIDS are highly susceptible to opportunistic infections and
cancers that take advantage of an immune system in collapse. Because AIDS arises from the loss of helper T cells, both
humoral and cell-mediated immune responses are impaired. The loss of helper T cells results from infection by the human
immunodeficiency virus (HIV).
AP Biology Notes Outline
Chapter 43: The Body’s Defenses
(1982) Describe the following mechanisms of response to foreign materials in the human body.
a. The antigen-antibody response to a skin graft from another person.
b. The reactions of the body leading to inflammation of a wound infected by bacteria
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(2005) An important defense against diseases in vertebrate animals is the ability to eliminate, inactivate, or destroy
foreign substances and organisms. Explain how the immune system achieves the following:
 Provides an immediate nonspecific immune response.
 Activates T and B cells in response to an infection.
 Responds to a later exposure to the same infectious agent.
 Distinguishes self from nonself
--------------------------------------------------------------------------------------------------------------------------------------------------(2007 B) The defenses of the human body to the entry and establishment of a pathogen (disease causing organism) can be
divided into nonspecific and specific responses.
a. Explain how THREE types of nonspecific defenses can prevent the entry and/or establishment of a pathogen in a
person’s body.
b. Discuss how the immune system responds to an initial pathogenic exposure, and how this initial exposure can lead
to a quicker response following a second exposure to the same pathogen.
c. Explain the biological mechanisms that lead to the rejection of transplanted organs.
--------------------------------------------------------------------------------------------------------------------------------------------------(1991) The graph below shows the response of the human immune system to exposure to an antigen. Use this graph to
answer part a and part b of this question.
a. Describe the events that occur during period I as the immune system responds to the initial exposure to the antigen.
b. Describe the events that occur during period II following a second exposure to the same antigen.
c. Explain how infection by the AIDS virus (HIV) affects the function of both T and B lymphocytes.
d. Explain the cause/effect of ONE autoimmune disease of your choice on the human body.