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Immunity
Our first line of defense against foreign organisms is barrier tissues such as the
skin (mechanical barrier) that stop the entry of organism into our bodies. If,
however, these barrier layers are penetrated, the body contains cells that respond
rapidly to the presence of the invader. Other mechanical barriers include:
-Movement of cilia in respiratory track
-Peristalsis in alimentary canal
-Flushing action of tears and saliva
-Acidic stomach to kill microorganisms 1
If these barriers did not stop the invaders (pathogens) then the blood would play
an important role in defending the body (second line of defense). E.g. phagocytes
(WBC) would engulf microbe and kill them, or lymphocytes would produce
antibodies that would attach to antigens (found on cell surface of pathogens) and
destroy the invading pathogen. Lymphocytes normally produce specific antibodies,
and also recall previous attacks of the body.
Immunity can be grouped into two categories
1. Natural Immunity
2. Artificial Immunity
Natural Immunity
a. Actively acquired immunity: Antibody-antigen reaction
-cause pathogen to clump together
-disintegrate pathogen, e.g. make it more porous by damaging cell membrane
-neutralize toxins produced by pathogens

Production of antibodies takes some time to occur, and pathogen would have
produced symptoms of the disease before they are destroyed by the
immune system.

However, if a person is infected a second time by the same pathogen, then
the antibodies production would be more rapid, since the lymphocytes would
have remembered the previous pathogenic attack
b. Passively acquired Immunity: This is a short lived immunity. It occurs during
breast feeding, where breast milk contains antibodies to protect the new born
against infections. Antibodies may also enter the feotus before birth through the
placenta.
Artificial Immunity
Vaccine
a. Actively acquired Immunity:
 This is mainly through vaccination. The vaccine contains weakened live
pathogen, or dead pathogens, that cannot cause the disease.

However, lymphocytes would still recognize them (i.e. the antigen on their
surface), and produce antibodies.

The next time a pathogen enters the body, the immune system
(lymphocyte) would immediately produce antibodies, since it already
"learned" the specific antigen from the vaccine.

This type of immunity may last for a short time or a lifetime. E.g. Typhoid
vaccines last three years, while tuberculosis vaccine lasts a lifetime2.
b. Passively Acquired Immunity:
This is also though vaccination. However the vaccine contains the specific antibody
or antitoxin that the body cannot make. This is a short lived immunity, since the
lymphocytes cannot learn to make them. E.g. Hepatitis vaccine last for three
months2.