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Course: Immunology
Lecturer: Dr. Weam Saad
Lecture: Innate Immunity
Innate (non-specific Immunity)
Human body is continuously exposed to pathogenic microorganisms.
The immune system is composed of two major subdivisions, the innate or
nonspecific immune system and the adaptive or specific immune system.
The innate immune system is our first line of defense against invading
organisms while the adaptive immune system acts as a second line of
defense and gives protection against re-exposure to the same pathogen. Each
of the major subdivisions of the immune system has both cellular and
humoral components by which they carry out their protective function and
help each other to do these functions.
The main function of the immune system is to distinguish between self
and non-self a protein which is necessary to protect the organism from
invading pathogens and to eliminate modified or altered cells (e.g.
malignant cells). Since pathogens may replicate intracellularly (viruses and
some bacteria and parasites) or extracellularly (most bacteria, fungi and
parasites), different components of the immune system have evolved to
protect against these different types of pathogens.
Innate Host defenses:
A. Anatomical barriers against infections:
1. Mechanical (physical ) factors
The epithelial surfaces form a physical barrier that is very impermeable
to most infectious agents. Thus, the skin acts as the first line of defense
against invading organisms. The shedding of skin epithelium also helps
remove bacteria and other infectious agents that have adhered to the
epithelial surfaces. Movement due to cilia or peristalsis helps to keep air
passages and the gastrointestinal tract free from microorganisms. The
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flushing action of tears and saliva helps prevent infection of the eyes and
mouth. The trapping affect of mucus that lines the respiratory and
gastrointestinal tract helps protect the lungs and digestive systems from
infection.
2. Chemical factors
Fatty acids in sweat inhibit the growth of bacteria. Lysozyme and
phospholipase found in tears, saliva and nasal secretions can breakdown the
cell wall of bacteria and destabilize bacterial membranes. The low pH of
sweat and gastric secretions prevents growth of bacteria.
3. Biological factors
The normal flora of the skin and in the gastrointestinal tract can prevent the
colonization of pathogenic bacteria by secreting toxic substances or by
competing with pathogenic bacteria for nutrients or attachment to cell
surfaces.
B. Humoral barriers against infections:
The anatomical barriers are very effective in preventing colonization of
tissues by microorganisms. However, when there is damage to tissues the
anatomical barriers are breeched and infection is occurs. Once infectious
agents have penetrated tissues, another innate defense mechanism comes
into play, namely acute inflammation. Humoral factors play an important
role in inflammation, which is characterized by edema and the activation of
phagocytic cells. These humoral factors are found in serum or they are
formed at the site of infection. They contain:
1. Complement system: The complement system is the major humoral
nonspecific defense mechanism (see the lecture of complement). Once
activated complement can lead to increased vascular permeability,
activation of phagocytic cells, and lysis and opsonization of bacteria.
2. Interferons: Interferons are proteins that can limit virus replication in
cells.
3. Lysozyme: Lysozyme breaks down the cell wall of bacteria.
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C. Cellular barriers against infections:
These cells are the main line of defense in the nonspecific immune system,
they include;
1. Polymorphonuclear cells (PMNs): Microphage, include Neutrophils,
Eosinophils and Basophils), they migrate to the site of infection
where they phagocytose invading organisms and kill them
intracellularly. In addition, PMNs contribute to tissue damage
that occurs during inflammation.
2. Monocytes and Macrophages: Macrophage, Tissue macrophages and
monocytes function is phagocytosis and intracellular killing of
microorganisms. In addition, macrophages are capable of
extracellular killing of infected or transformed cells (self-
target). Also, macrophages have role in tissue repair and act as
antigen presenting cells APC, which are required for the
induction of specific immune responses.
Phagocytosis: A very important process during non-specific immune
response when specialized cells engulf foreign body like bacteria or molecule
like toxin or virus. The phagocytosis has four steps:
1. Chemotaxis.
2. Endocytosis.
3. Phagolysosome formation and degradation of foreign substances.
4. Lysis and excretion.
The cells that able to do phagocytosis are (monocytes, macrophage, PMNs
and Dendric cells). The results of phagocytosis are:
1. Complete destruction of foreign body and excretion (PMNs)
2. Complete destruction of foreign body and some parts (polypeptides) of
it will be processed and presented on the surface of the phagocytic cells
(monocytes, macrophage and dendric cells) then the phagocytic cell will be
antigen presenting cell (APC).