The Nature of Host Defense pdf - D. Dhafer

Lecture 1: The nature of host defense Dr Dhafer Alghezi

Introduction:

Immunology: is the study of all biological, chemical, and physical events surrounding the

function of the immune system (to react) against infectious microbes (bacteria- viruses-

fungi- parasites) and non-infectious foreign substances to eliminate them to protect the

body. The term immunity means the state of protection from infectious disease involving

specific and non-specific elements. The immune system is a complex collection of fluids

and cells that penetrate every organ, tissue space, fluid compartment, and vascular network

of the body.

Our environment contains a great variety of infections microbes. These can cause disease,

and if they multiply unchecked, they will eventually kill their host. So, the primary function

of the immune system is to eliminate infectious agents and to minimize the damage they

cause.

The protection of our organism against infectious agents involves many different

mechanisms: some nonspecific (i.e., generically applicable to many different pathogenic

organisms) and others specific (i.e., their protective effect is directed to one single

organism).

Immune response:

Any immune response involves, firstly, recognition of the pathogen or other foreign

material, and secondly a reaction to eliminate it. The different types of the immune

response fall into two categories:

Innate (non-specific) immune response which is the first line of defense against infection

and it does not alter on repeated exposure to a given infectious agent and it react in the

same manner with each pathogen.

Adaptive (specific) immune response: it is a highly specific for a particular pathogen and

the immune response improves with each successive encounter with the same antigen.

Thus, the two-key feature of the adaptive immune response are the specificity and the

memory.

Lecture 1: The nature of host defense Dr Dhafer Alghezi

The table below shows the main differences between innate and adaptive immune

responses.

N0. Characteristics

Innate Immunity

Adaptive immunity

Presence

Innate immunity is something already

present in the body.

Adaptive immunity is created in

response to exposure to a foreign

substance.

Specificity

Non-Specific

Specific

Response

Fights any foreign invader.

Fight only specific infection.

Response

Rapid

Slow (1-2 weeks)

Potency

Limited and Lower potency.

High potency

Time span

Once activated against a specific type

of antigen, the immunity remains

throughout the life.

The span of developed immunity can

be lifelong or short.

Inheritance

Innate type of immunity is generally

inherited from parents and passed to

offspring.

Adaptive immunity is not passed

from the parents to offspring; hence

it cannot be inherited.

Memory

Cannot react with equal potency upon

repeated exposure to the same

pathogen.

Adaptive system can remember the

specific pathogens which have

encountered before.

Presence

Present at birth

Develops during a person’s lifetime

and can be short-lived.

Used Against

For microbes

Microbes

and

non-microbial

substances called antigens

Memory

No memory

Long term memory

Complement

system

activation

Alternative and lectin pathways

Classical pathway

Anatomic and

physiological

barriers

Skin, Mucous membranes, Temp, pH,

chemicals, etc.

Lymph nodes, spleen, mucosal

associated lymphoid tissue.

Composition

The innate immune system is

composed of physical and chemical

barriers,

phagocytic

leukocytes,

Adaptive

immune

system

composed of B cells and T cells.

Lecture 1: The nature of host defense Dr Dhafer Alghezi

dendritic cells, natural killer cells, and

plasma proteins.

Example

White blood cells fighting bacteria,

causing redness and swelling, when

you have a cut.

Chickenpox vaccination so that we

don’t get chickenpox because

adaptive immunity system has

remembered the foreign body.

Defenses development and specificity

There are three lines of defenses against any foreign bodies: 1st line such as chemical,

physical and genetic barriers, second line defenses include phagocytosis, inflammation,

complement system, and interferon and third line defenses: acquired immunity.

First line defense:

The natural, inborn, nonspecific defenses that can be divided into physical, chemical, and

genetic barriers that impede the entry of microbes at the site of first contact.

Lecture 1: The nature of host defense Dr Dhafer Alghezi

Barriers at the Body’s Surface

In the skin

• (stratum corneum) of the skin is composed of epithelial cells that have become

compacted, cemented together, and impregnated with an insoluble protein, keratin.

• Sebaceous secretions exert an antimicrobial effect

• The high lactic acid and electrolyte concentrations of sweat and the skin’s acidic pH and

fatty acid content

Eye

• Blinking and (lacrimation) flush the eye’s surface with tears and rid it of irritants.
• Specialized glands such as the meibomian glands of the eyelids lubricate the

conjunctiva with an antimicrobial secretion (lysozyme)

• Lysozyme: an enzyme that hydrolyzes the peptidoglycan in the cell wall of bacteria.

The digestive tract

• The mucocutaneous membranes of the free surface of digestive tract, in addition to

saliva secretion (lysozyme) impedes the entry of bacteria

• The constant flow of saliva helps carry microbes into the harsh conditions of the

stomach.

• The HCl in the stomach renders protection against many pathogens that are swallowed.

• the intestine’s digestive juices and bile are potentially destructive to microbes.

• Vomiting and defecation also evacuate noxious substances or microorganisms from the

body.

Respiratory tract effective adaptations

• Nasal hair traps larger particles.

• Flushing action and copious flow of mucus and fluids that occurs in allergy and colds.

• The ciliated epithelium (called the ciliary escalator) in the respiratory tree a conveys

foreign particles entrapped in mucus toward the pharynx to be removed

• Irritation of the nasal passage reflexly initiates a sneeze, which expels a large volume of

air at high velocity.

• Similarly, the acute sensitivity of the bronchi, trachea, and larynx to foreign matter

triggers coughing, which ejects irritants.

Lecture 1: The nature of host defense Dr Dhafer Alghezi

The genitourinary tract

• The mucocutaneous membranes of the free surface of genitourinary tract

• periodic bladder emptying and continuous trickle of urine through the ureters that

flushes the urethra.

• Semen contains an antimicrobial chemical that inhibits bacteria, and the vagina has a

protective acidic pH maintained by normal flora.

Non-anatomical barriers

like resident flora by creating an unfavorable environment

for pathogens like pH in intestine and vagina.

Genetic Defenses

Some pathogens have great specificity to infect one host species that they are incapable of

infecting other species (e.g. Humans can’t acquire distemper from cats, and cats can’t get

mumps from humans). Genetic differences insusceptibility can also exist within members

of one species. Humans carrying a gene or genes for sickle-cell anemia are resistant to

malaria. why?

Q) Why the first line does not consider a true immune response?

because it does not involve recognition of a specific foreign substance but is very general

in action.

The importance of first line of defenses is manifested by examples:

1- Patients with severe skin damage due to burns are extremely susceptible to infections

2- With blockages in the salivary glands, tear ducts, intestine, and urinary tract are also at

greater risk for infection.

Q) Why the first line of defense alone is not sufficient to protect against infection?

Because many pathogens find a way to circumvent the barriers by using their virulence

factors.

Lecture 1: The nature of host defense Dr Dhafer Alghezi

Immune terminology

pathogen: an infectious agent that causes disease

Infectious disease: occurs when microorganism succeeds in evading host defenses to

establish a local site of infection and replication.

Foreign material: Is something that can be recognized or distinguished as not being a

natural part of an organism’s body

Surveillance: Is the process of scouting the tissues for foreign molecules and other

possibly threatening particles

Recognition: Is the process of evaluating and differentiating of the molecules that detect

by white blood cells surveillance.

Markers: Are molecules that protrude from the cell surface like minuscule signposts

announcing that cell or molecule’s identity.

Specificity: A unique configuration that dictates the kinds of immune responses it can

elicit.

Receptors: Molecules that bind specifically with complementary molecules in ways that

signal, communicate, and trigger reactions inside the cell.

Antigen: A foreign, non-self-molecule, often a surface marker, that evokes a specific

immune response.