Cell Surface Markers

Foundations in Microbiology

Sixth Edition

Chapter 15Adaptive, Specific

Immunity and Immunization

Lecture PowerPoint to accompany

Talaro

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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

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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

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• Natural active immunity – acquired upon infection and recovery

• 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

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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

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Development of the Immune Response System

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

• Major functions of receptors are:– To perceive and attach to nonself or foreign

molecules– To promote the recognition of self molecules– To receive and transmit chemical messages

among other cells of the system– To aid in cellular development

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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

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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

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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

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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

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• 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.

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• 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

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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

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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.

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T-Cell Receptors for Antigen

• Formed by genetic recombination, with variable and constant regions

• 2 parallel polypeptide chains

• small, without humoral functions

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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

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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.

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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

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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, epitope – small molecular group that is recognized by lymphocytes

• Antigen has many antigenic determinants.

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• Foreign molecules less than 1,000 MW, called haptens

• Not antigenic unless attached to a larger carrier

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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

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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 TH cells.– Interleukin-2 is produced by TH to activate B and other T

cells.

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B-cell Activation and Antibody Production

• Once B cells process the Ag, interact with TH 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.

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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.

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Antibody-Antigen InteractionsPrinciple 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 – Abs fill the surface receptors on a 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.

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Functions of the Fc Fragment

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

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Classes of Immunoglobulins5 classes of immunoglobulins (Ig):

– IgG– IgA– IgM– IgD– IgE

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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.

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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

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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

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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.

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Types of T cells• T helper cells (CD4 or TH) 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 (TH1) cells or T helper 2 (TH2) cells

• Cytotoxic T cells (CD8 or TC) destroy foreign or abnormal cells by secreting perforins that lyse cells.

• Natural killer cells – lack specificity; circulate through the spleen, blood, and lungs

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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

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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.

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Vaccine Preparation

Most vaccines are prepared from:

• Killed whole cells or inactivated viruses

• Live, attenuated cells or viruses

• Antigenic molecules derived from bacterial cells or viruses

• Genetically engineered microbes or microbial agents

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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

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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

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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.

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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

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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

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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.

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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