NTA Level 5 • Semester 1 • PST05103

Antigens and Antibodies

Pharmaceutical Microbiology • Source Session/Topic 46

Full source-text version: all educational wording from the extracted learning source is retained; only presenter/tutor metadata and web-layout noise are removed, while formatting is improved for readability.

Session 46: Antigens and Antibodies

Total Session Time: 120 minutes

Pre-requisites

• Human anatomy and physiology

Students Learning Tasks

By the end of this session students are expected to be able to:

• Differentiate between antigens and antibodies

• Explain sources of antibodies

• Classify antibodies

Resources Needed:

• Flip charts, marker pens, and masking tape

• Black/white board and chalk/whiteboard markers

SESSION OVERVIEW

SESSION CONTENTS

STEP 1: Presentation of Session Title and Learning Tasks (5 minutes)

READ or ASK students to read the learning tasks and clarify

ASK students if they have any questions before continuing

STEP 2: Antigens (35 minutes)

Definition of Antigens

• Antigens are substances that induce a specific immune response and

subsequently react with the products of a specific immune response.

• An antigen is a molecule that stimulates an immune response.

• The definition encompasses all substances that can be recognized by

the adaptive immune system.

• Antigens differ from allergens

• An allergen is a substance that causes the allergic reaction. The

(detrimental) reaction may result after exposure via ingestion,

inhalation, injection or contact with skin

• The name “antigen” is derived from the term “ANTI’body GEN’erator”

• Superantigens are class of molecules that can interact with antigen

presenting cells and T-cells in nonspecific way e.g. staphylococcal

exotoxins, Toxic shock syndrome toxins and some viral proteins

Classification of Antigens

• Antigens can be classified in order of their origins into exogenous

antigens, endogenous antigens and autoantigens.

o Exoantigens

▪ Exogenous antigens are antigens that have entered the body

from the outside, for example by inhalation, ingestion, or

injection.

▪ These antigens enter the body by endocytosis or phagocytosis

• The antigens are taken into the antigen-presenting cells

(APCs) and processed into fragments.

o Antigen-presenting cells (APCs) are a heterogeneous

group of immune cells that mediate the cellular immune

response by processing and presenting antigens for

recognition by certain lymphocytes such as T cells.

Classical APCs include dendritic cells, macrophages,

Langerhans cells and B cells.

o Endogenous antigens

▪ Endogenous antigens are antigens that have been generated

within the cell, as a result of normal cell metabolism, or

because of viral or intracellular bacterial infection.

o Autoantigens

▪ An autoantigen is usually a normal protein or complex of

proteins (and sometimes DNA or RNA) that is recognized by the

immune system of patients suffering from a specific autoimmune

disease.

▪ These antigens should under normal conditions not be the

target of the immune system, but due to mainly genetic and

environmental factors the normal immunological tolerance for

such an antigen has been lost in these patients.

o There are antigens that are known as tumor antigens

▪ Tumor antigens are those antigens that are presented by the

major histocompatibility complex class I (MHC I) molecules on

the surface of tumor cells.

▪ These antigens can sometimes be presented only by tumor cells

and never by the normal ones. In this case, they are called

tumor-specific antigens (TSAs) and typically result from a tumor

specific mutation.

• Antigens can also be classified as Thymus-dependent antigens (TD-Ag)

and Thymus independent antigens (TI-Ag)

• Characteristics of Antigens

o Immunogenicity

▪ The capacity to stimulate the production of antibodies or cell-

mediated immune responses

▪ It is the ability of the antigen to stimulate an immune

response from the immune system

o Antigenicity

▪ The ability to bind antibody

▪ An incomplete antigen that cannot elicit an immune response is

known as a hapten

o Foreignness

▪ this an antigen being recognized as non-self. It is important

feature because self-responsive cells are eliminated during

lymphocyte activation leaving only cells that respond to non-

self

o Specificity

▪ This is the uniqueness of an antigen.

▪ Antigen specificity depends on epitopes

• An epitope is the region or part of the antigen that is

active immunogically i.e. it binds to antibodies and

products of immune response

o Chemical and structural complex

▪ Proteins are the most potent immunogens

▪ Polysaccharides are less immunogenic

▪ Nucleic acids and lipids do not elicit good immune response

▪ Molecules with simple chemical structures e.g. a simple sugar

are less immunogenic than complex molecules

o Molecular size

▪ Large molecular weight proteins are highly antigenic

▪ Smaller molecular weight molecules are less antigenic

o Stability

▪ Highly stable and non-degradable substance are not immunogenic

o Biological systems

▪ Some substances are immunogenic in one individual but not

immunogenic in others

o Dose of antigen

▪ Very low dose does not stimulate immune response

▪ Repeated accumulation may be needed to stimulate immune

response in some cases e.g. booster doses in immunization such

as in diphtheria

o Route of entry

▪ Antigens entering through parenteral route produce good levels

of antibodies

o Adjuvants

▪ A substance that increases immunogenicity of an antigen when

mixed with that antigen and then injected

▪ Adjuvants increase the strength and duration of immune

response

• Aluminium potassium sulfate

STEP 3: Antibodies (35 minutes)

• An antibody (or an Immunoglobulin) is a specialized immune protein

produced because of the introduction of an antigen into the body, and

which possesses the ability to combine with the very antigen that

triggered its production.

• The production of antibodies is a major function of the immune system

and is carried out by B-lymphocytes

• Antibodies can be triggered by and directed at foreign proteins,

microorganisms, or toxins

• Some antibodies are autoantibodies interact against the body’s own

tissues

• Antibodies identify and neutralize pathogens such as bacteria and

viruses or toxic substances

Characteristics of antibodies

• Antibodies are immunoglobulin (Ig) molecules

• Antibodies are antigen specific and bind to foreign molecules

• They are produced by activated B-cells

• Antibodies are first molecules participating in specific immune

response

• They mediate effector function to neutralize or eliminate foreign

invaders

Structure of Antibodies

• Antibody molecules have a common structure of four peptide chains.

• This structure consists of two identical light (L) chain polypeptide

of about 22000 Da and two identical heavy (H) chain of larger

polypeptide of about 55000 Da or more.

• The identical heavy chains and two identical light chains connected by

disulfide bonds

• An antibody is made up of a variable region and a constant region,

• The variable region changes to various structures depending on

differences in antigens

o Antigens (epitope) bind to this region of antibody

• The constant region is the same always

[pic]

STEP 4: Classification of Antibodies (25 minutes)

• Antibodies are classified into five classes

o Immunoglobulin G (IgG)

▪ Make up approximately 80% of the serum antibodies

▪ They cross the placenta and enter foetal blood circulation

▪ Their functions include;

• Provide immunity to new born babies

• Neutralize toxins

o Immunoglobulin M (IgM)

▪ They make up approximately 13% of the total serum antibodies

▪ They are the first antibodies to be produced in primary

response to antigen

▪ Functions

• Activation of the classic pathway (of the complement

system)

• Defend against multivalent antigens

• Act as opsonins

o Immunoglobulin A (IgA)

▪ They make up approximately 6% of the serum antibodies

▪ Are found mainly in body secretions such as saliva, mucous,

tears, colostrum and milk

• Functions

o Act as secretory antibodies

o Effective against Influenza virus

o Produced in infant gut

o Provide first defense for mucosal surfaces such as the

intestines, nose, and lungs

o Immunoglobulin D (IgD)

▪ They make approximately 0.2% of the serum antibodies

▪ Are found on the surface of B-lymphocytes

▪ Functions

• Activates B-cells

o Induce antibody production

• Act as receptors for antigen binding

o Immunoglobulin E (IgE)

▪ It is in very low concentration

▪ Contain small percentage of lymphocytes

▪ Functions

• Responsible for immediate hypersensitivity reactions

• Binds to basophils and mast cells

• They release substances like histamine

STEP 5: Differences Between Antigens and Antibodies (10 minutes)

• The differences between antigens and antibody

|Antibody |Antigen |

|Generally proteins but can be |Antibodies are proteins |

|lipids, carbohydrates or | |

|nucleic acids | |

|Triggers the formation of |Variable sites has the antigen |

|antibodies |binding domain |

|There three basic kinds of |There are five basic kinds of |

|antigens (exogenous, |antibodies (Immunoglobulins M, G, |

|endogenous and autoantigens) |E, D and A) |

|The region of the antigen that|The variable region of the antibody|

|interacts with antibodies is |that specially binds to an epitope |

|called |is called paratope |

|Cause disease or allergic |Protects the body by immobilization|

|reactions |or lysis of antigenic material |

STEP 6: Key Points (5 minutes)

• Antigens stimulate the immune system to produce immune response

• Antigens stimulate specific immunity

STEP 7: Evaluation (5 minutes)

• What is an antigen?

• What are the differences between antigen and antibody?

• What are the classes of antibodies?

References

Hugo and Russell (2011), Pharmaceutical Microbiology 8th Edition, Willey-

Blackwel publications

Karen C. Carroll et al (2013); Jawetz, Melnick and Adelberg’s Medical

Microbiology 26th Ed. McGraw Hill Co. Inc.

Greenwood et al (2012); Medical Microbiology, 18th edition Churchill

Livingstone

← Previous Topic Next Topic →View all Pharmaceutical Microbiology topics Open Complete Full Notes

PDF / OFFLINE NOTES

Unataka kutumiwa notes hizi kupitia WhatsApp?Kwa notes zilizopangiliwa vizuri kwa kusoma offline au PDF, bonyeza kitufe hapa chini. Ujumbe wenye Level, Semester, Module na Topic utaandaliwa moja kwa moja.TUMIWA NOTES WHATSAPP

WhatsApp: 255620339260