Introduction and Overview of the immune system – Immunity Section

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Introduction and Overview of the immune system


A)   What is immunity or what is the immune system?

==>Immunity or the immune system is a collection of biological processes within an organism that:

  • Protects against disease by identifying and killing pathogens (from viruses to parasitic worms) and tumor cells
  • Distinguish the organism’s own healthy cells and tissues from the pathogens in order to function properly.

==> Detection is complicated because pathogens can evolve rapidly; producing adaptations that avoid the immune system and allow the pathogens to successfully infect their host, therefore the immune system has multiple mechanisms evolved to recognize and neutralize pathogens.

B)   Degree of complexity درجة تعقيده:

Vertebrates such as humans have more sophisticated defence mechanisms than other organisms because the immune systems of vertebrates consist of many types of proteins, cells, organs, and tissues, which interact in an elaborate مدروس ودقيق and dynamic network.

C)   How to recognize specific pathogens:

==> As part of this more complex immune response, the human immune system adapts over time to recognize specific pathogens more efficiently and this is called “adaptive immunity” or “acquired immunity” and creates immunological memory to a specific pathogen from the primary encounter (infection for the 1st time),

==>  This immunological memory provides an enhanced response to secondary encounters (infection for te 2nd time) with that same, specific pathogen. (This process of acquired immunity is the basis of vaccination).



A)   Types of the immune system:
The immune system is composed of two major subdivisions,

  • The innate or non­-specific immune system                and
  • The adaptive or specific immune system.

Innate immune system

Adaptive immune system

first line of defence

a second line of defence

Has cellular and humoral components

Has cellular and humoral components

has anatomical features that function as barriers to infection like the skin

Doesn’t has anatomical features

React immediately and ready to be mobilized upon infection because it includes defenses that, for the most part, are constitutively present (doesn’t require lag period)

requires some time to react to an invading organism (requires lag period)

is not antigen specific and reacts equally well to a variety of organisms

antigen specific and reacts only with the organism that induced the response

Does not demonstrate immunological memory.

demonstrates immunological memory to react rapidly for the 2nd encounter (affords protection against re-exposure to the same pathogen)

Let’s explain these diferences

1) Why the innate immune system is the first line and what is the innate immune system?

==> The innate immune system is attacked first by all pathogens so he is not specific to one enemy … for example, the skin protects the body from invasion of any pathogen like bacteria, viruses …etc …. and if the pathogen succeed invading the skin, a signal will be transmitted to the adaptive immune system to get ready to recognize that pathogen and prepare the specific weapon …. and for this reason the adaptive is the second line  because he take time to prepare specific weapon but the innate start defense immediately and with no specific weapon.

==> This is such as the army of your country…. each army has Frontier troops which will attack any enemy immediately with the weapon they have and if the enemy can invade the borders, the Frontier troops will notify the leadership to be ready and attack with more advanced and specific weapons.

2) Both of them has cells and also proteins that works as hormonal signals

3) The innate immune system has anatomical features because it may be an organ like skin but the adaptive immune system has cells not organs

4) The innate immune system doesn’t demonstrate immunological memory because he attack any enemy so he doesn’t need to mempreize each pathogen but the adaptive immune system recognize and identify each pathogen to make specific weapons so he make a memory for this enemy to be ready and attack faster in the second attack

B) The cellular component of the immune system:
==> All cells of the immune system is created from the stem cell  of the bone marrow

==> There are 2 types of stem cells in the bone marrow:

1) The myeloid cells (= myeloid progenitor):

It is differentiate into:
==> Granulocyte: which differentiate into: 

  • Neutrophils
  • Basophils.
  • Eosinophils,

==> Monocyte:

  • Macrophages
  • Dendritic cells
  • Kuffer cells (special type of macrophage present in the liver).

2. lymphoid cells (lymphoid progenitor):

  • Natural Killer
  • B lymphocyte which is then differentiated into the plasma cell which then forms the antibodies.
  • T lymphocyte: which must migrate to the thymus where they undergo differentiation into two distinct types of T cells:
  • The CD8+ T-pre-cytotoxic cell                      and
  • The CD4+ T-helper cell which differentiate into Two types
    • The TH1 cells, which help the CD8+ pre-cytotoxic cells to differentiate into cytotoxic T cells,
    • The TH2 cells, which help B cells to differentiate into plasma cells, which secrete antibodies


C) The main function of the immune system:

The main function of the immune system is self/non-self discrimination which is necessary to protect the organism from invading pathogens and to eliminate modified or altered cells (e.g. malignant cells).

D) The reason for the complexity of the immune system:

Different components of the immune system have evolved to protect against the different types of mechanisms by which the pathogens invade the body where there are:

  • Pathogens may replicate intracellularly (viruses and some bacteria and parasites)   or
  • Extracellularly (most bacteria, fungi and parasites).

E) When the disease occur in presence of the immune system:

The infection with an organism does not necessarily mean diseases because the immune system inmost cases will be able to eliminate the infection before disease occurs, therefore when the disease occur??????

Disease occurs only when:

==> The bolus of infection is high

==> The virulence of the invading organism is great

==> When immunity is compromised.

F) The harmful effect of the immune system:
Although the immune system, for the most part, has beneficial effects, there can be detrimental effects as well such as:

  • During inflammation, which is the response to an invading organism, there may be local discomfort and collateral damage to healthy tissue as a result of the toxic products produced by the immune response.
  • In addition, in some cases the immune response can be directed toward self tissues resulting in autoimmune disease.

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