AP Biology Notes Outlinechapter 43: the Body S Defenses

AP Biology Notes OutlineChapter 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.

1. CELLULAR INNATE DEFENSES combat pathogens that get through the skin. They include phagocytic WBCs and antimicrobial proteins:
2. Neutrophils: WBCs that ingest and destroy microbes via phagocytosis.
3. Monocytes: WBCs that migrate into tissue and develop into macrophages, giant phagocytic cells.
4. Eosinophils: WBCs that defend against parasitic invaders such as worms by discharging hydrolytic enzymes.
5. Interferon: an antimicrobial protein against viral infections – causes cells adjacent to infected cells to produce substances to inhibit viral replication.
6. Complement System: consists of ~30 proteins with a variety of functions – including lysing invading cells.

1. LOCAL INFLAMMATORY RESPONSE – triggered by damage to tissue or the entry of pathogens.
2. 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.
3. Histamines trigger the dilation and permeability of nearby capillaries – this dilation aids in delivering clotting agents and phagocytic cells to the injured area.

1. 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.

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.

1. 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.

1. 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.

1. ANTIBODIES: soluble proteins secreted by B cells during an immune response.

1. B or T CELL ACTIVATION: occurs when an antigen binds to a B or T cell.

1. 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.
2. Effector Cells: combat the antigen.
3. Memory Cells: are long-lived, bear receptors for the same antigen, and can quickly mount an immune response in subsequent infections.

1. ANTIGEN BINDING: B cell receptors bind intact antigens, and T cell receptors bind antigens that are displayed by antigen-presenting cells on the MHCs.

1. 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:
2. Class I MHCs – found on almost all cells of the body, except RBCs.
3. Class II MHCs – made by dendritic cells, macrophages, and B cells.

1. 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!

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. 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.
2. 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.

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 Tc is bound to the infected cell’s class I MHC by its CD8 protein. Tc cells destroy infected body cells.

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 antibody-secreting 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 2nd time.

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.

Active & Passive Immunization

Active immunity develops naturally in response to an infection or it can develop following immunization, also called vaccination. In immunization, anonpathogenic 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 arthritisis 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 immunodeficiencyresults 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).

(1982) Describe the following mechanisms of response to foreign materials in the human body.

1. The antigen-antibody response to a skin graft from another person.
2. 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

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(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.

1. Explain how THREE types of nonspecific defenses can prevent the entry and/or establishment of a pathogen in a person’s body.
2. 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.
3. Explain the biological mechanisms that lead to the rejection of transplanted organs.

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(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.

1. Explain how infection by the AIDS virus (HIV) affects the function of both T and B lymphocytes.
2. Explain the cause/effect of ONE autoimmune disease of your choice on the human body.