Host Response to infection

1. HOST RESPONSE
TO INFECTION

2. LEARNING OUTCOMES
 Discuss the antigen and immunogen
 Explain the properties and types of antigen
 Describe the different lines of defense of the body
 Differentiate innate immunity and adaptive immunity
 Examine the functions of the different immunoglobulins
 Explain the role of vaccines in the prevention of the
development of infectious diseases.

3. ANTIGEN
Antigen is a substance recognize by the immune system,
whether by the B cell or the T cell that serves as the target
of the immune response but may not necessarily lead to an
immune response.
Antigens are large molecules of proteins, present on the
surface of the pathogen- such as bacteria, fungi viruses,
and other foreign particles. When these harmful agents
enter the body, it induces an immune response in the body
for the production of antibodies.

4. Properties of Antigens
The properties of antigens are as follows:
1. The antigen should be a foreign substance to induce an immune
response.
2. The antigens have a molecular mass of 14,000 to 6, 00,000 Da.
3. They are mainly proteins and polysaccharides.
4. The more chemically complex they are, the more immunogenic they
will be.
5. Antigens are species-specific.
6. The age influences the immunogenicity. Very young and very old
people exhibit very low immunogenicity.

5. Types of Antigens
On the basis of Origin
There are different types of antigens on the basis of origin:
1. Exogenous Antigens
-Exogenous antigens are the external antigens that enter the body from outside,
e.g. inhalation, injection, etc. These include food allergen, pollen, aerosols, etc. and
are the most common type of antigens.
2. Endogenous Antigens
-Endogenous antigens are generated inside the body due to viral or bacterial
infections or cellular metabolism.
3. Autoantigens
- Autoantigens are the ‘self’ proteins or nucleic acids that due to some genetic or
environmental alterations get attacked by their own immune system causing
autoimmune diseases.

6. On the basis of Origin
There are different types of antigens on the basis of origin:
4. Tumour Antigens
-It is an antigenic substance present on the surface of
tumour cells that induces an immune response in the host,
e.g. MHC-I and MHC-II. Many tumours develop a
mechanism to evade the immune system of the body.
5. Native Antigens
-An antigen that is not yet processed by an antigen-
presenting cell is known as native antigens.

7. On the Basis of Immune Response
On the basis of the immune response, antigens can be classified
as:
 Immunogen
-These may be proteins or polysaccharides and
can generate an immune response on their own.
 Hapten
-These are non-protein, foreign substances that
require a carrier molecule to induce an immune
response.

8. Structure of Antigens
The epitopes or antigenic determinants are the components
of antigen. Every antigen has several epitopes. An antibody
has at least two binding sites that can bind to specific
epitopes on antigens. The antigens combine with the
antibody according to the lock and key mechanism. The
ability of the body to act against the disease-causing agents
and antigens by the immune system is termed as the
immunity. This immunity may be either inborn or acquired
from vaccinations.

9. Immune response
● The immune response is how your body recognizes and defends itself
against bacteria, viruses, and substances that appear foreign and
harmful.
● The immune system protects the body from possibly harmful substances
by recognizing and responding to antigens. Antigens are substances
(usually proteins) on the surface of cells, viruses, fungi, or bacteria.
Nonliving substances such as toxins, chemicals, drugs, and foreign
particles (such as a splinter) can also be antigens. The immune system
recognizes and destroys, or tries to destroy, substances that contain
antigens.

10. INNATE IMMUNITY
● Innate immunity is also known as natural immunity. This immunity is already
active from the time of birth, prior to exposure to an antigen. Innate immunity is
non- specific. It includes host barriers that prevent entry of microorganisms
such as the skin and mucous membranes (first line of defense), and
processes such as phagocytosis and information (second line defense)
which prevents the multiplication of organisms that gain entry to the baby
preventing them from multiplying before they have a chance to produce
disease. It is activated with minutes following exposure to the antigen.
However, it does not improve after exposure to the antigen and does not
possess memory and thus provides only short- term protection.

11. Important Components of Innate Immunity
Factors that limit entry of microorganisms Mode of action
Keratin layer of intact skin
Lysozyme in tears and other secretions
Respiratory cilia
Low pH of stomach and vagina; fatty acids in skin
Surface phagocytes
Act as mechanical barrier
Degrades bacterial cell wall
Directs organisms trapped in mucus out of
the respiratory passages
Inhibits growth of microorganism’s
Ingest and destroy microbes
Prevent colonization by pathogens
Factors that limit growth of the microorganisms
within the body
Mode of action
Natural killer cells
Neutrophils
Macrophages and dendritic cells
Interferon’s
Complement system
Fever
Inflammatory response
Kills virus-infected cells
Ingest and destroy microbes
Ingest and destroy microbes; present
antigens to T
cells
Inhibit viral replication; produce anti-viral state
Membrane attack complex creates holes in
bacteria
cell membrane; components activate
inflammation
Inhibits bacterial growth
Limits spread of microbes by destroying them

12. ADAPTIVE IMMUNITY
Adaptive immunity is specific. It is
activated after to a particular antigen.
Unlike innate immunity, it is an acquired
response to an antigen that is initiated
by recognition of specific epitopes of
the foreign invaders

13. Property Innate immunity Adaptive Immunity
Activity at birth Yes No
Response time Immediate delayed
Specificity for
microorganism
Relatively low (PAMPs) High(specific antigens)
Cells Phagocytic cells, NK
cells, epithelial cells
B lymphocytes and T
lymphocytes
Memory No Yes
Response amplified No Yes
Components
Physical and chemical
barriers
Blood proteins
Line of defense
Skin, mucosa;
antimicrobial
substances (e.g..,
defensing)
Complement
First and second
Secreted antibodies
antibodies
Line
Third ( immune
response)

14.  ACQUIRED IMMUNITY
-Acquired immunity is immunity that develops with exposure to various antigens.
Your immune system builds a defense against that specific antigen.
 PASSIVE IMMUNITY
- Passive immunity is due to antibodies that are produced in a body other than your
own. Infants have passive immunity because they are born with antibodies that are
transferred through the placenta from their mother. These antibodies disappear
between ages 6 and 12 months.
-Passive immunization may also be due to injection of antiserum, which contains
antibodies that are formed by another person or animal. It provides immediate
protection against an antigen, but does not provide long-lasting protection. Immune
serum globulin (given for hepatitis exposure) and tetanus antitoxin are examples of
passive immunization.
●

15. First Line of Defense
● The primary defense against infectious disease are the
surface barriers that prevent pathogens from entering the
body
● These surface barriers include intact skin (protect external
boundaries) and mucous membranes (protect internal
boundaries)
● Both the skin and mucous membranes release chemical
secretions which restrict the growth of microbes on their
surfaces
● If pathogens cannot enter the host body, they cannot
disrupt normal physiological functions and cause disease

16. Second Line of Defense
● The second line of defense against infection are the non-specific
cellular and molecular responses of the innate immune system
● These defenses do not differentiate between different types of
pathogen and respond the same way upon every infection
● Phagocytic leukocytes migrate to infection sites and engulf foreign
bodies (dendritic cells then present antigens to lymphocytes)
● Inflammatory responses increase capillary permeability at infected
sites, recruiting leukocytes but leading to localized swelling
● Antimicrobial proteins (such as cytokines and complement
proteins) regulate immune activity within the body
● Fever increases body temperatures to activate heat-shock proteins
and suppress microbial growth and propagation

17. Third Line of Defense
● The final line of defense against infection are the
lymphocytes that produce antibodies to specific antigenic
fragments
● Each B cell produces a specific antibody, and the body has
millions of different B cells capable of detecting distinct
antigens
● Helper T cells regulate B cell activation, ensuring that
antibodies are only mass-produced at the appropriate times
● Both B and T cells will differentiate to form memory cells
after activation, conferring long-term immunity to a particular
pathogen

18. ANTIBODIES
Antibodies are globulin (immunoglobulins) that
react specifically with the antigens that stimulate
their production. The most important functions of
anti-bodies are (1) to neutralize toxins and viruses;
(2) to opsonize microbes so that they will be readily
recognized and more easily phagocytosed; (3) to
activate complement system; and (4) to prevent the
attachment of microbes to mucosal surfaces.

19. ANTIBODY STRUCTURE

20. CLASSES OF ANTIBODIES
There are five classes of anti bodies found in all species and all individuals. Each
immunoglobulin class is defined by its component heavy chain.
1. IgG- a monomer and is the predominant antibody in the secondary immune
response (anamnestic) response) and is a major defense against bacteria and viruses.
2. IgM- the largest among the immunoglobulins and is a pentamer.
3. IgA- called the secretory immunoglobulin and is the main immunoglobulin in
secretions such as colostrum, saliva, and tears, as well as respiratory, gastrointestinal,
and genitourinary tract secretions.
4. IgE- also called the reaginic antibody. It is medically important for two reasons (1) it
mediates immediate or anaphylactic hypersensitivity reaction and (2) it provides
defense against parasites such as helminths or worms.
5. IgD- a monomer that has no known antibody function. It is found on the surface of
many B cells and serves as the surface maker for B cells but may also function as an
antigen receptor.

21. The following table summarizes the properties of
immunoglobulins and illustrates their basic structures:

22. VACCINES
A great number of infectious diseases can be prevented by administering vaccines that induce
either active or passive immunity. It contains a weakened (also called attenuated) or
inactivated form of organism). It may contain the entire organism or a specific portion of the
microbe (also known as sub- unit). It may also be derived from toxins produced by the
microorganisms (toxoids). Whatever the component , vaccines are designed to stimulate the
body’s immune system to produce the antibodies specific to the organism or it components so
that these are recognized as foreign and will be destroyed immediately upon entry of the
organisms into the body. The immunization of a population stops spread of an infectious agent
by reading the number of susceptible hosts (herd immunity). Immunization programs have
achieved the following goals:
1. Protection of population group from the development of common infectious such as
pertussis, diphtheria, mumps, and rubella
2. Control of the spread of measles, mumps, and rubella
3. Elimination of smallpox in world

23. Types of immunization
● Passive immunization involves the administration of purified antibody in preparation
called immune globulins or antibody-containing serum. It is given for rapid, temporary
protection (usually 3-4 months) or treatment of a person (eg., in the treatment of rabies).
The protection given is short-lived. It is used with the following goals in mind: (1) to prevent
disease after a known exposure; (2) to reduce the symptoms of an ongoing disease; (3) to
protect immunosuppressed patients; or (4) to block the action of bacterial toxins and
prevent the disease that they cause. Immune serum globulin preparations are derived
from infected humans or animals and are available as prophylaxis for several bacterial and
viral diseases.
● Active immunization involves the injection of vaccines prepared from organism or their
products. This stimulates the body’s immune system to produce the specific antibodies
against the component organism of the vaccine. The response takes days to weeks to
develop but the protection given is long-term or even lifelong. Active immunization is
preferred over passive immunization.

24. Types of vaccine
● Live attenuated vaccines are prepared using organism with limited
ability cause disease. These are especially useful for protection
against infection caused by enveloped viruses.
● Toxoid vaccines were developed based on the principle that certain
disease are caused by exotoxins produced by the causative agents.
● Killed vaccines refer to vaccines derived from bacterial sources while
inactivated vaccines are derived from viruses.
● Subunit vaccine- this type of vaccine is produce the say way as the
killed/ inactivated vaccine but instead of using the entire organism as
the antigen to stimulate anti body production, only a specific antigen
or structure on the organism is used.

25. THANK YOU!!!
PREPARED BY: RONNA MAE FARINAS BSED SCIENCE 3A