1. Chapter 21 Nonspecific Body Defenses and Immunity
G.R. Pitts, J.R. Schiller, and James F. Thompson, Ph.D.

2. Defense Systems Innate (nonspecific) defenses
External body membranes
Inflammation
Antimicrobial proteins, phagocytes and other cells
Adaptive (specific) defenses
T cells and B cells

3. Innate Defense System Surface Barriers Internal Nonspecific Defenses
First line of defense: mechanical and chemical protection
Skin
Mucosal Membranes
Internal Nonspecific Defenses
Second line of defenses
Phagocytes
Natural Killer cells (NK lymphocytes)
Inflammation
Antimicrobial proteins
Fever

4. Skin and Mucosal Membranes
Mechanical Protection
Epidermis
nose hairs, nails
Mucous membranes - line certain organ systems
mucus prevents drying, traps foreign things
respiratory tract cilia sweep mucus out
Lacrimal apparatus -- tear glands and ducts
wash the eye to dilute microbial growth
Saliva - dilute microbes on the oral cavity
Urine - flow dilutes, and acid pH helps kill, microorganisms
Defecation and vomiting - expel toxins and microbes

5. Skin and Mucosal Membranes
Chemical Protection: reduce bacterial growth
Skin
sebum (unsaturated FA’s) forms oily layer
perspiration has fatty acids, salts (NaCl), and mildly acid pH
Lysozyme
in perspiration, tears, saliva, nasal secretions, other tissue fluids
enzyme breaks down bacterial cell walls
Hyaluronic acid
gel-like matrix in most connective tissues
slows the spread of many infectious agents
Gastric juice - stomach nearly sterile due to acid pH, ~2
Vaginal secretions – mildly acid pH

6. Innate Defense: Phagocytes
Macrophages (derived from monocytes) are the chief tissue phagocytic cells
Free macrophages wander through tissues in search of microbes and cellular debris
Fixed macrophages: Kupffer cells (liver), microglia (brain), dust cells (lungs)
Neutrophils become phagocytic when encountering infectious material
Eosinophils are weakly phagocytic, deploy destructive granules against parasitic worms

7. Mechanism of Phagocytosis
Chemotaxis
Adherence – recognition of external carbohydrates and proteins
Aided by opsonins
Ingestion
Killing and digestion

8. Innate Defense: Natural Killer Cells
Distinct group of large granular lymphocytes (NK lymphocytes = Null Killer lymphocytes)
Nonspecific killers respond to the lack of self-antigens and to the presence of certain surface oligosaccharides
Kill virus-infected body cells and some tumor cells by releasing various defensive molecules – not by phagocytosis
Act before the antigen-specific immune system is activated
Secrete potent chemical signals that enhance the inflammatory response

9. Innate Defense: Inflammation
Signs:
Redness
Heat
Swelling
Pain
Loss of Function
Function:
Prevent spread of damage
Dispose of pathogens and debris
Set stage for tissue repair

10. Inflammation Stage 1: Vasodilation and increased vessel permeability
Macrophages and cells lining the gastrointestinal and respiratory tracts carry Toll-Like Receptors (TLRs) that recognize specific classes of microbes
TLReceptor activation causes cytokine release
promotes inflammation & chemotaxis
Mast cells secrete histamine
Other cells secrete various regulatory factors
Histamine, kinins, prostaglandins, leukotrienes, complement
Cause local vasodilation
Increase capillary permeability resulting in edema

11. Inflammation: Stage 1
Edema – increased plasma filtrate seeps into tissue spaces bringing some immune proteins
Helps to dilute harmful substances
Increases supply of oxygen and nutrients needed for metabolism, inflammation and repair
Allows entry of clotting proteins, which reduces the spread of mibrobes

12. Inflammation Stage 2. Phagocyte moblization
Leukocytosis-inducing factors: increase neutrophil production
Margination (pavementing)
Diapedesis (amoeboid movement)
Chemotaxis of WBCs
neutrophils – rapid arrival
monocytes – slower arrival

13. Inflammation Stage 3. Tissue repair
Tissue regrowth and repair of damage or scar formation
Pus
dead phagocytes and other WBCs, damaged tissue, and perhaps microbes
if too numerous for effective removal by phagocytes, an abscess may develop

14. Effects of Inflammation
Increased blood flow results in increased local temperature and local cellular metabolism
Increased capillary permeability and phagocytic migration to the injured tissue

15. Innate Defense: Antimicrobial Proteins
Attack microorganisms directly
Interfere with microbial reproduction
The most important are:
Interferons
The Complement System
Transferrins which bind Fe2+ in plasma, inhibiting bacterial growth

16. Interferons (IFNs)
Produced by most tissue cells when infected by a virus
Diffuses to uninfected cells and binds to surface receptors
stimulates macrophages and natural killer lymphocytes
stimulates production of antiviral proteins which block viral replication
inhibits growth of virally infected cells
suppresses growth of tumor cells
Alpha IFN is used against:
hepatitis C virus
herpes virus (genital warts)

17. The Complement System
20 plasma and cell membrane proteins that exist as inactive precursors
When activated, the complement system functions to “complement” or enhance certain immune, inflammatory, and allergic responses
Kills bacteria and certain other microbial cell types (our cells normally are protected from complement attack)
Stimulates chemotaxis in leuckocytes
Enhances the effectiveness of both nonspecific and specific defenses

18. Complement Pathways
Classical Pathway is triggered by the specific immune system
Requires binding of antibodies to antigens of invading organisms
Complement C1 then binds to the antigen-antibody complexes (complement fixation)
Alternative Pathway is triggered by non-specific interaction among factors B, D, and P, and microbial cell wall polysaccharides (complement fixation)
Both pathways involve an enzyme cascade

19. Complement Pathways
Both pathways converge on C3, which cleaves into C3a and C3b
C3b initiates formation of a membrane attack complex (MAC)
MAC causes cell lysis by creating many hundreds of microscopic holes in the cell’s plasmalemma
C3b is also an opsonin

20. Innate Defense: Fever High fevers are dangerous
Pyrogens reset the temperature set-point in the hypothalamus
Inhibits some microbes from growing
Increases body’s metabolic rate, which speeds up immune defenses and tissue repair
Increases effects of antimicrobial substances produced by the immune system
Stimulates liver and spleen to sequester iron and zinc (needed by microorganisms)
High fevers are dangerous

21. Innate Defense System: Review
Surface Barriers
Skin
Mucosal membranes
Internal Nonspecific Defenses
Phagocytes
Natural Killer cells (NK lymphocytes)
Inflammation
Antimicrobial proteins
Fever

22. Adaptive Defense The adaptive immune system:
Acts to immobilize, neutralize, or destroy foreign substances and cells
Amplifies the inflammatory response and activates complement
Is antigen-specific*, systemic, and has memory
*Recognizes specific foreign molecules
Has two interdependent arms
Humoral, or antibody-mediated immunity (AMI)
Cellular, or cell-mediated immunity (CMI)

23. Adaptive Defense Definitions:
Immunity: the ability of the body to defend itself against specific foreign invaders (molecules or cells)
Immunogenicity: the ability to stimulate proliferation of specific lymphocytes and specific antibody production
Reactivity: the ability of activated lymphocytes and their products, antibodies, etc., to interact with specific antigens

24. Adaptive Defense Definitions:
Specificity: the antigen triggers focused immune defenses (from particular lymphocytes lineages) that respond only to the antigens of this foreign substance/cell
Memory: the immune system produces clones of specific memory lymphocytes (T & B) which react rapidly when the particular foreign substance/cell is encountered again
Specificity and memory differentiate this system from the nonspecific (innate) defenses

25. Adaptive Defense
Antigen – any substance which provokes specific immune responses
Antigenic determinants
Parts of antigens that trigger the specific immune response
An antigen may be an entire microorganism or only small structures or subregions of large molecules
Most “antigens” are complex and express multiple types of antigenic determinants.

26. Chemical Nature of Antigens
“Complete” Ag: complex macromolecules - usually proteins (nucleo-, lipo-, glyco-) -- sometimes carbohydrates or lipids
Are immunogenic & reactive
“Incomplete” Ag: smaller molecules (haptens)
react with antibodies but cannot cause an immune response without aid (protein carrier)
e.g., poison ivy, drug allergies

27. Adaptive Defense Antigen receptor diversity
>1 billion different antigenic determinants are recognized by the body
Genetic recombination shuffles and reorganizes different Ab genes
Major histocompatibility complex antigens (MHC)
unique to each individual’s cells; help in identifying what is self versus foreign
2 classes of MHC antigens (“markers”)
class I MHC – found on all body cells except RBC's
class II MHC - only on antigen presenting cells (APC’s), thymus cells, and activated T cells

28. Antigen-Presenting Cells (APCs)
APCs phagocytize, process, and present antigens to lymphocytes
APCs do not respond to specific antigens
APCs contribute to coordinating specific immunity
Macrophages
Dendritic (Langerhans) cells
B lymphocytes
The major initiators of adaptive immunity are APCs, which actively migrate to the lymph nodes and secondary lymphoid organs and present antigens to T and B cells

29. Class I MHC Proteins Found on all cells, except RBCs
Recognized by T lymphocytes and APCs
Display peptides from endogenous antigens
Endogenous antigens are:
Associated with body cells
Degraded by proteases and enter the endoplasmic reticulum
Transported through special membrane channels
Bound with MHC class I molecules on the ER membrane
Migrate to the cell membrane as a complex: Ag -- MHC class I molecule

30. This is a form of Antigen Presentation
MHC Class I Proteins
This is a form of Antigen Presentation
Cancer cells often do something quite similar to the virus-infected cells.
(Foreign MHC Class I Ags are the source of tissue transplant rejections.)

31. MHC Class II Proteins
Immune cell identity markers found only on mature B cells, some T cell classes, and antigen-presenting cells
MHC Class II proteins are synthesized in the ER
A phagosome containing a pathogen (with exogenous antigens) merges with a lysosome
MHC Class II proteins migrate into the phagosome where the antigen macromolecules are degraded and particular antigen peptides are bound to the MHC Class II markers
Ag-- MHC class II complex then migrates to the cell membrane and displays antigenic peptide for recognition by CD4 TH cells

32. This is a key function of our APCs in most Ag-specific defenses.
MHC Class II Proteins
This is a key function of our APCs in most Ag-specific defenses.

33. Lymphocytes Provide Ag Specificity
B and T lymphocytes develop in bone marrow
Lymphocytes mature and develop immunocompetence (ability to recognize specific antigen) in different locations
B cells mature in the bone marrow and provide Ab-mediated immunity
T cells mature in the thymus and provide cell-mediated immunity

34. Immunocompetent B or T cells
Naive cells display a unique surface receptor for a specific antigen once mature
Receptor expression occurs before a cell encounters the foreign antigen it may later attack
It is genes, not antigens, that determine which foreign substances our immune system will recognize and resist
Naive cells circulate to secondary lymphoid tissue where they may encounter antigens later
B and T cells become fully functional only after binding with their recognized antigen

35. Immunocompetent T Cells
Survive
Apoptosis
T cells mature in the thymus under positive and negative selection pressures
Positive selection – outer thymic cortex
Selects functional T cells which become both immunocompetent and potentially self-tolerant
Non-selected cells die via apoptosis
Negative selection – inner thymic cortex
Kill or regulate off T cells that react with self-antigens

36. Immunocompetent B Cells
B cells become immunocompetent and self-tolerant in bone marrow
Some self-reactive B cells are killed by apoptosis (clonal deletion)
Some self-reactive B cells can modify their anti-self properties (receptor editing)
Some self-reactive B cells are released from the bone and are inactivated by negative regulation (anergy)

37. Cell-Mediated Immunity
CMI is involved in most aspects of specific immune defense
Three populations of T lymphocytes regulate specific immunity
Helper TH cells which carry CD4+ markers
Suppressor TS cells
Memory T cells
cytotoxic TC cells which carry CD8+ markers destroy tumor cells and virus-infected cells; they also attack transplanted cells and tissues

38. Cell -Mediated Immunity
Basic steps
Recognition by T lymphocytes of antigen presented by an antigen-presenting cell with matching MHC Class II markers
Proliferation and differentiation of T cells once activated
Production of clones of identical effector T cells capable of recognizing a specific antigen
Appropriate action (help, attack, memory, suppression) from T cell subclones

39. T Cell Activation- Step 1: Antigen Binding and Antigen Presentation

40. T Cell Activation- Step 2: Co-Stimulation
T cells must bind to MHC Class II surface receptors on an APC
After co-stimulation with cytokines, T cells enlarge, proliferate, and form clones
Activated T cells differentiate and perform functions according to their T cell class

41. T Lymphocyte Activity
Primary T cell response usually peaks within a week
T cells then undergo apoptosis within a month
Reduced activity parallels elimination of antigen
This is a negative feedback control
A few Memory T cells remain to respond to any future exposure to the same antigen

42. Helper TH Lymphocytes
Regulatory cells that play a central management role in the immune response
Once primed by APC antigen presentation, TH cells:
Stimulate proliferation of other T cell classes
Stimulate B cells that have already become bound to antigen
There is NO coordinated immune response without TH cell function

43. Helper TH Lymphocytes
TH cells interact directly with B cells that have antigen fragments on their surfaces bound to MHC Class II receptors
TH cells express CD4+ cell identity markers
TH cells stimulate B cells to divide more rapidly and begin antibody formation
B cells may be activated without TH cell help by binding to T cell–independent antigens (certain microbial polysaccharides)
Most antigens, however, require TH co-stimulation to activate B cells
Cytokines released by TH amplify nonspecific defenses

44. Cytotoxic Tc Lymphocytes
TC cells express CD8+ cell identity markers
TC cells, or killer T cells, are the only T cells that can directly attack and kill other cells
They circulate throughout the body in search of body cells that display the antigen to which they have been sensitized
Their targets include:
Virus-infected cells
Cells with intracellular bacteria or parasites
Cancer cells
Foreign cells from blood transfusions (WBCs and platelets) or tissue and organ transplants

45. Cytotoxic Tc Lymphocytes
Bind to self/anti-self complexes on any body cell
Infected or abnormal cells can be destroyed as long as appropriate antigen and co-stimulatory regulators (e.g., IL-2) are present
[In contrast, Natural Killer cells activate their killing machinery when they bind to a different MHC-related cell surface marker on cancer cells, virus-infected cells, and transplanted cells]

46. Cytotoxic Tc Lymphocyte Actions
Secrete perforins which cause cell lysis by creating transmembrane pores
Secrete lymphotoxin which fragments the target cell’s DNA
Secrete gamma interferon which stimulates macrophage attack

47. Suppressor Ts Lymphocytes
TS cells – immune regulatory cells which release cytokines that suppress the activity of both T cells and B cells
Generated when other specific T cell clones are generated
Negative feedback control to bring the body back to normal after the “battle” has been won

48. Antibody-Mediated Immunity
Antigen challenge – the first encounter between an antigen and a naive B lymphocyte
Antigen presentation usually occurs in the spleen or a lymph node, but can occur in any lymphoid tissue
Antigen presentation usually made by a macrophage, but some B cells can react directly against certain bacterial antigens
Binding of the antigen to the B cell’s specific Ag receptor activates the B cell

49. Primary Response
Activated B cells grow and divide, forming clones bearing the same antigen-specific receptors and secreting the same antigen-specific Ab
Most clone cells become plasma cells that secrete specific antibodies
Clones that do not become plasma cells become B memory cells that can respond to subsequent exposures to the same antigen

50. Primary Response
Initial B cell differentiation, proliferation, and Ab synthesis requires time after the first Ag exposure
Lag period: 3 to 6 days after antigen challenge
Peak plasma levels of antibody are achieved in ~10 days
Antibody molecules also reach the interstitial fluids, especially where inflammation exists
Antibody levels then decline gradually if there is no additional Ag exposure

51. Secondary Response Any subsequent exposure to the same antigen
Sensitized memory cells (B and T) respond within hours
Antibody levels peak in 2 to 3 days at higher plasma levels than in the primary response
Activated B subclones generate antibodies that bind with greater affinity
Plasma antibody levels can remain high for weeks to months

52. Primary and Secondary Antibody Responses

53. Immunological Memory
Immunization is possible because memory B cells and memory T cells persist after the initial Ag exposure
with any subsequent exposure, the immune system responds more quickly, forcefully
secondary response - antibodies produced during subsequent exposures are produced in greater quantities and have a greater attraction for antigen

54. Antibodies
Are unique soluble proteins secreted by activated B cells and plasma cells in response to an antigen
Are capable of binding specifically with that antigen
Constitute much of the gamma globulin fraction of plasma proteins
Also called immunoglobulins

55. Basic Antibody Structure
Four polypeptide chains linked together with disulfide bonds
The four chains bound together form an antibody monomer
Each chain has a variable (V) region at one end and a constant (C) region at the other
Variable regions of the heavy and light chains combine to form the antigen-binding site Ag

56. Antibody Structure
Antibodies responding to different antigens have different V regions but the C region is the same for all antibodies in a given antibody class
C regions form the stem of the Y-shaped antibody monomer and determine:
the class of the antibody
the cells and chemicals to which the antibody can bind
how an antibody class functions in eliminating antigens

57. Classes of Antibodies
IgD: monomer attached to the surface of B cells, important in B cell activation
IgM: pentamer released by plasma cells during the primary immune response
IgG: monomer that is the most abundant and diverse antibody in primary and secondary responses; crosses the placenta and confers passive immunity
IgA: dimer that helps prevent attachment of pathogens to mucosal surfaces
IgE: monomer that binds to mast cells and basophils, causing histamine release when activated

58. Antibody Functions
All antibodies form an antigen-antibody (immune) complex
Antibodies do not directly destroy antigen, though they may immobilize or inactivate Ag
Antibodies act as opsonins and tag Ag for immune attack and destruction
Defensive mechanisms triggered by antibodies include neutralization, agglutination, precipitation, opsonization, and complement fixation

59. Antibody Mechanisms of Action
Neutralization: Antibodies bind to and block specific sites on viruses or exotoxins, thus preventing these antigens from binding to receptors on tissue cells
Antibodies bind to the same determinant on more than one antigen forming antigen-antibody complexes that are cross-linked into large lattices
Agglutination: Cellular antigens are cross-linked, causing cell clumping
Precipitation: Soluble molecules are cross-linked into large insoluble complexes

60. Antibody Mechanisms of Action
Opsonization: Bound Abs facilitate phagocyte adherence
Complement Fixation: IgM and IgG antibodies bound to cellular Ags bind complement via the Classical Pathway
The complement cascade causes chemotaxis, opsonization, phagocytosis and cell lysis
Complement activation enhances the inflammatory response

61. Summary of Antibody Actions
Figure 21.13

62. Monoclonal Antibodies
Monoclonal antibodies are purified tissue culture preparations of a specific antibody for a single antigenic determinant which are produced from descendents of a single B cell
Commercially prepared monoclonal antibodies are used:
To provide passive immunity
In research applications
In clinical laboratory testing
In the treatment of certain cancers

63. Adaptive Immunity: Summary
A defensive system with two interdependent arms (CMI & AMI) that uses lymphocytes, APCs, and specific molecules to recognize and destroy foreign substances
Adaptive immune responses depend on the ability of its cells to:
Distinguish foreign from self molecules
React with foreign substances (antigens) by binding to them
Communicate with one another to effect a coordinated protective response specific to those antigens

64. Adaptive Immunity: Summary
To start an immune response, APCs, B and T lymphocytes must recognize foreign antigen
Antigen-Presenting Cells and some B cells recognize and immediately bind to certain antigens in the blood, the extracellular fluid (ECF), or other tissue spaces
More often, B and T cells only recognize antigen (protein fragments) when Ag is presented by the macrophages in combination with MHC Class II surface markers and stimulation is provided by Th lymphocytes

65. Summary of the Immune Response

66. Clinical Classification of Immunity
Active Immunity: the body’s own B and T lymphocytes encounter antigens and produce specific responses against them; immunological memory does occur
Naturally Acquired – response to a microbial or parasitic infection
Artificially Acquired – response to a vaccine of dead or attenuated (weakened) pathogens
Passive Immunity: An outside source of immune cells or molecules is provided to a recipient; immunological memory does not occur; protection ends when the donated materials are naturally eliminated from the body
Naturally Acquired – the mother to her baby via the placenta (IgG) or via lactation (colostrum/milk) (IgM & IgA)
Artificially Acquired – the injection of serum, gamma globulin, or leukocyte transfusion

67. Clinical Classification of Immunity

68. Organ and Tissue Transplants
The four major types of grafts are:
Autograft – graft transplanted from one site on the body to another in the same person
Isograft – graft between identical twins (or clones); individuals with the same genotype
Allograft – graft between individuals that are not identical twins, but belong to same species
Xenograft – grafts taken from another animal species

69. Prevention of Graft Rejection
Donors are selected to minimize differences in MHC Class I antigens = HLA (human leukocyte antigens)
Unnecessary for routine blood transfusions since RBCs lack HLAs
Prevention of rejection is accomplished by using various immunosuppressive drugs
Survival and longevity of grafts have varying success
Immunosuppressive drugs depress the patient’s immune system so it is less effective in defending against pathogens and cancer

70. Pathologies: Immunodeficiencies
Human Immunodeficiency Virus
HIV enters certain cell types by receptor mediated endocytosis
infects primarily helper T cells
attaches to the CD4 protein on cell surface
A retrovirus
carries its genetic material as RNA
inserts its genetic material into host cell DNA with the enzyme reverse transcriptase
cell makes copies of the virus, releases them for further infection
May be carried silently in cells for years, being passed on during ordinary mitosis
Activation of HIV life cycle destroys THelper cells
Weakened immune response to all foreign invaders, benign or aggressive

71. Pathologies: Autoimmune Diseases
Multiple Sclerosis (MS) – myelin sheath (white matter) attacked and destroyed
Myasthenia Gravis – ACh receptors at neuro-muscular junction of skeletal muscle attacked and destroyed
Grave’s Disease – thyroid cells’ TSH receptor attacked and stimulated causing excess thyroid hormone (T3 & T4) production
Type I Diabetes - destruction of pancreatic islet cells eliminates insulin secrection

72. Pathologies: Autoimmune Diseases
Systemic Lupus Erythematosus (SLE) – generalized attack on connective tissues and nuclear antigens
Glomerulonephritis - destruction of the glomerular capillaries causes impaired renal function
Rheumatoid Arthritis - destruction of the synovial membranes in joints

73. Pathologies: Cancer
The immune system probably evolved first to respond to cancer cells
when a new cancer cell develops, new surface marker proteins (tumor antigens) often appear
if the immune system recognizes these new surface markers as non-self, it will destroy the cell expressing them
this immune surveillance is most effective in eliminating virus-induced tumor cells because they tend to express viral antigens which are not “self”
Leukemias and Lymphomas – cancers of leukocytes

74. Pathologies: Hypersensitivities
Immediate hypersensitivities (allergies)
First exposure merely sensitizes one to an allergen (penicillin, venoms, dust, mold, pollen, etc.)
APCs digest and inappropriately present the allergen
Subclones of B cells secreting IgE predominate in response
Anti-allergen IgE attaches to mast cells and basophils
Later exposures produce dramatic responses
Antigen binds to IgE on mast cells and basophils
Ag-IgE binding triggers these cells to release much histamine and other inflammatory molecules
Local reactions – swelling, rashes, erythema, itching
Systemic reactions – asthma, anaphylactic shock, death

75. Pathologies: Subacute Hypersensitivities
Caused by IgG and IgM
Occurs 1-3 hr after exposure and lasts hr
Cytotoxic reactions
Ab bind to Ag on specific cells causing phagocytosis and complement-activated lysis
May occur after transfusion of mismatched blood
Immune-complex hypersensitivities
Ag’s are widely distributed or insoluble Ag-Ab complexes can’t be removed
Intense inflammation
Severe damage to local tissue
Also involved in autoimmune diseases

76. Pathologies: Delayed Hypersensitivities
Occurs 1-3 days after exposure
Cell-mediated immune response
Causes mild swelling to serious cytotoxic tissue damage (contact dermatitis, e.g., TB skin test, poison ivy, latex gloves, etc.)
[Note: Sometimes allergies may be temporarily transferred by blood or plasma transfusions.]

77. End Chapter 21