Lecture PowerPoint to accompany

Foundations in Microbiology

Sixth Edition Talaro Chapter 15 Adaptive, Specific Immunity and Immunization Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Specific Immunity – Adaptive Line of Defense

Third line of defense – acquired • Production of specific antibodies by dual system of B and T lymphocytes in response to an encounter with a foreign molecule, called an

antigen

• Two features that characterize specific immunity: – –

specificity

– antibodies produced, function only against the antigen that they were produced in response to

memory

– lymphocytes are programmed to “recall” their first encounter with an antigen and respond rapidly to subsequent encounters 2

Classifying Immunities

• • • •

Active immunity

– results when a person is challenged with antigen that stimulates production of antibodies; creates memory, takes time and is lasting

Passive immunity

– preformed antibodies are donated to an individual; does not create memory, acts immediately, and is short term.

Natural immunity

– acquired as part of normal life experiences

Artificial immunity

- acquired through a medical procedure such as a vaccine 3

• • • •

Natural active immunity

infection and recovery – acquired upon

Natural passive immunity

– acquired by a child through placenta and breast milk

Artificial active immunity

– acquired through inoculation with a selected Ag

Artificial passive immunity

– administration of immune serum or globulin 4

5

Overview of Specific Immune Responses

• • • • • Separate but related activities of the specific immune response: Development and differentiation of the immune system Lymphocytes and antigens The challenge of B and T lymphocytes by antigens B lymphocytes and the production and activities of antibodies T lymphocyte responses 6

7

Development of the Immune Response System

Cell receptors or markers confer specificity and identity of a cell.

• Major functions of receptors are: 1. To perceive and attach to nonself or foreign molecules 2. To promote the recognition of self molecules 3. To receive and transmit chemical messages among other cells of the system 4. To aid in cellular development 8

Major Histocompatibility Complex (MHC)

• Receptors found on all cells except RBCs • Also known as human leukocyte antigen (HLA) • Plays a role in recognition of self by the immune system and in rejection of foreign tissue 9

Functions of MHC

• Genes for MHC clustered in a multigene complex: – Class I – markers that display unique characteristics of self molecules and regulation of immune reactions • required for T lymphocytes – Class II – receptors that recognize and react with foreign antigens; located primarily on macrophages and B cells • involved in presenting antigen to T-cells 10

Lymphocyte Receptors

• Lymphocyte’s role in surveillance and recognition is a function of their receptors.

• B-cell receptors – bind free antigens • T-cell receptors – bind processed antigens 11

Clonal Selection Theory

• Lymphocytes use 500 genes to produce a tremendous variety of specific receptors.

• Undifferentiated lymphocytes undergo genetic mutations and recombinations while they proliferate in the embryo.

• Form a billion different clones with the ability to react with a tremendous variety of antigens 12

• In the bone marrow, lymphocytic stem cells differentiate into either T or B cells.

• B cells stay in the bone marrow.

• T cells migrate to the thymus.

• Both T and B cells migrate to secondary lymphoid tissue.

13

14

• Lymphocyte specificity is preprogrammed, existing in the genetic makeup before an antigen has ever entered the system.

• Each genetically different type of lymphocyte expresses a single specificity.

• First introduction of each type of antigen into the immune system selects a genetically distinct lymphocyte.

• Causes it to expand into a clone of cells that can react to that antigen 15

16

Specific B-Cell Receptor: Immunoglobulin

Receptor genes of B cells govern immunoglobulin (Ig) synthesis.

• Large glycoproteins that serve as specific receptors of B cells • Composed of 4 polypeptide chains: – 2 identical heavy chains (H) – 2 identical light chains (L) • Y shaped arrangement – ends of the forks formed by light and heavy chains contain a wide range of variable

antigen binding sites

• Variable regions • Constant regions 17

18

Development of Receptors

• Immunoglobulin genes lie on 3 different chromosomes • Undifferentiated lymphocyte has 150 different genes for the variable region of light chains and 250 for the variable region and diversity region of the heavy chain • During development, recombination causes only the selected V and D genes to be active in the mature cell.

• Once synthesized, immunoglobulin is transported to cell membrane and inserted there to act as a receptor.

19

20

T-Cell Receptors for Antigen

• Formed by genetic recombination, with variable and constant regions • 2 parallel polypeptide chains • small, without humoral functions 21

22

B-cell Maturation

• Directed by bone marrow sites that harbor stromal cells, which nurture the lymphocyte stem cells and provide hormonal signals • Millions of distinct B cells develop and “home” to specific sites in the lymph nodes, spleen, and GALT • Come into contact with antigens throughout life 23

T-Cell Maturation

• Maturation is directed by the thymus gland and its hormones.

• 7 classes of T-cell receptors termed CD -

cluster of differentiation

• Mature T cells migrate to lymphoid organs and occupy specific sites.

24

Entrance and Processing of Antigens and Clonal Selection

•

Antigen

(Ag) is a substance that provokes an immune response in specific lymphocytes.

• Also called an

immunogen

• Property of behaving as an antigen is

antigenicity

25

Characteristics of Antigens

• Perceived as foreign, not a normal constituent of the body • Foreign cells and large complex molecules over 10,000 MW are most antigenic.

• Antigenic determinant, lymphocytes

epitope

– small molecular group that is recognized by • Antigen has many antigenic determinants .

26

27

• Foreign molecules less than 1,000 MW, called

haptens

• Not antigenic unless attached to a larger carrier 28

29

Special Categories of Antigens

• • • •

Alloantigens

– cell surface markers of one individual that are antigens to another of that same species

Superantigens

– potent T cell stimulators; provoke an overwhelming response

Allergen

– antigen that provokes allergy

Autoantigens

– molecules on self tissues for which tolerance is inadequate 30

Antigen Processing and Presentation to Lymphocytes

• T-cell dependent antigens must be processed by phagocytes called antigen presenting cells (APC).

• APCs modify the antigen so it is more immunogenic and recognizable; then the Ag is moved to the APC surface and bound to MHC receptor.

• Antigen presentation involves a direct collaboration among an APC, a T helper cell and an antigen-specific B or T cell.

– Interleukin-1 is secreted by APC to activate T H – Interleukin-2 is produced by T H cells.

cells.

to activate B and other T 31

32

B-cell Activation and Antibody Production

• Once B cells process the Ag, interact with T H cells and are stimulated by growth and differentiation factors, they enter the cell cycle in preparation for mitosis and clonal expansion.

• Divisions give rise to plasma cells that secrete antibodies and memory cells that can react to the same antigen later.

33

34

Antibody Structure and Functions

• Immunoglobulins • Large Y-shaped protein • Consist of 4 polypeptide chains • Contains 2 identical fragments (Fab) with ends that bind to specific antigen • Fc binds to various cells and molecules of the immune system.

35

36

Antibody-Antigen Interactions

• • • Principle antibody activity is to unite with the Ag to call attention to, or neutralize the Ag for which it was formed.

Opsonization

– process of coating microorganisms or other particles with specific antibodies so they are more readily recognized by phagocytes

Agglutination

– Ab aggregation; cross-linking cells or particles into large clumps

Neutralization

virus or the active site on a microbial enzyme to prevent it from attaching – Antitoxins are a special type of Ab that neutralize a bacterial exotoxin.

– Abs fill the surface receptors on a 37

38

Functions of the Fc Fragment

• Fc fragment binds to cells – macrophages, neutrophils, eosinophils, mast cells, basophils, and lymphocytes.

39

40

Classes of Immunoglobulins

5 classes of immunoglobulins (Ig): 1. IgG 2. IgA 3. IgM 4. IgD 5. IgE 41

42

Antibodies in Serum

• If separated by electrophoresis, globulin separates into 4 bands: • Alpha-1 (α-1), alpha-2 (α-2), beta (β), and gamma (γ) • Most are antibodies.

• γ is composed primarily of IgG; β and α-2 are a mixture of IgG, IgA, and IgM.

43

44

Primary and Secondary Responses to Antigens

• •

Primary response

– after first exposure to an Ag immune system produces IgM and a gradual increase in Ab

titer

(concentration of antibodies) with the production of IgG

Secondary response

–after second contact with the same Ag, immune system produces a more rapid, stronger response due to memory cells –

anamnestic response

45

46

Monoclonal Antibodies

• Pure preparation of antibody • Single specificity antibodies formed by fusing a mouse B cell with a cancer cell.

• Used in diagnosis of disease, identification of microbes and therapy 47

48

T Cells & Cell Mediated Immunity

• Cell mediated immunity requires the direct involvement of T lymphocytes.

• T cells act directly against Ag and foreign cells when presented in association with an MHC carrier.

• T cells secrete cytokines that act on other cells.

• Sensitized T cells proliferate into long-lasting memory T cells.

49

Types of T cells

1. T helper cells

(CD4 or T H ) most prevalent type of T cell; regulate immune reaction to antigens, including other T and B cells; also involved in activating macrophages and improving opsonization; differentiate into T helper 1 (T H 1) cells or T helper 2 (T H 2) cells

2. Cytotoxic T cells

or abnormal cells by secreting perforins that lyse cells.

(CD8 or T C ) destroy foreign

3. Natural killer

cells – lack specificity; circulate through the spleen, blood, and lungs 50

51

Immunization: Manipulating Immunity

•

Passive immunity

– immune serum globulin (ISG), gamma globulin, contains immunoglobulin extracted from pooled blood;

immunotherapy

• Treatment of choice in preventing measles and hepatitis A and in replacing antibodies in immunodeficient patients • Sera produced in horses are available for diphtheria, botulism, and spider and snake bites.

• Acts immediately; protection lasts 2-3 months 52

Vaccination

•

Artificial active immunity

– deliberately exposing a person to material that is antigenic but not pathogenic • Principle is to stimulate a primary and secondary anamnestic response to prepare the immune system for future exposure to a virulent pathogen • Response to a future exposure will be immediate, powerful, and sustained.

53

Vaccine Preparation

Most vaccines are prepared from: 1. Killed whole cells or inactivated viruses 2. Live, attenuated cells or viruses 3. Antigenic molecules derived from bacterial cells or viruses 4. Genetically engineered microbes or microbial agents 54

55

Killed or Inactivated Vaccines

• • Cultivate the desired strain, treat it with formalin or some other agent that kills the agent but does not destroy its antigenicity.

Often require a larger dose and more boosters to be effective 56

Live Attenuated Cells or Viruses

• Process that substantially lessens or negates the virulence of viruses or bacteria – eliminates virulence factors • Advantages of live preparations are: – organisms can multiply and produce infection (but not disease) like the natural organism – They confer long-lasting protection.

– usually require fewer doses and boosters • Disadvantages include: – require special storage, can be transmitted to other people, can conceivably mutate back to virulent strain 57

Antigenic Molecules

•

Acellular

or

subcellular vaccines (subunit

– if a virus) • Exact antigenic determinants can be used when known: – capsules – pneumococcus, meningococcus – surface protein – anthrax, hepatitis B – exotoxins – diphtheria, tetanus • Antigen can be taken from cultures, produced by genetic engineering, or synthesized. 58

Genetically Engineered Vaccines

• Insert genes for pathogen’s antigen into plasmid vector, and clone them in an appropriate host.

– stimulated the clone host to synthesize and secrete a protein product (antigen), harvest and purify the protein – hepatitis • “Trojan horse” vaccine – genetic material from a pathogen is inserted into a live carrier nonpathogen; the recombinant expresses the foreign genes – experimental vaccines for AIDS, herpes simplex 2, leprosy, tuberculosis 59

Genetically Engineered Vaccines

• DNA vaccines – create recombination by inserting microbial DNA into plasmid vector • Human cells will pick up the plasmid and express the microbial DNA as proteins causing B and T cells to respond, be sensitized, and form memory cells.

– experimental vaccines for Lyme disease, hepatitis C, herpes simplex, influenza, tuberculosis, malaria 60

61

Route of Administration ad Side Effects

• Most administered by injection; few oral, nasal • Some vaccines require

adjuvant

to enhance immunogenicity and prolong retention of antigen.

• Stringent requirements for development of vaccines • More benefit than risk • Possible side effects include local reaction at injection site, fever, allergies; rarely back-mutation to a virulent strain, neurological effects.

62

Herd Immunity

• Immune individuals will not harbor it, reducing the occurrence of pathogens –

herd immunity.

• Less likely that an nonimmunized person will encounter the pathogen 63