8. HUMAN HEALTH AND
DISEASES
HUMAN IMMUNE SYSTEM
|
§ It is the system that gives immunity to the body
by recognizing, responding and remembering foreign antigens. § It
plays role in allergic reaction, auto-immune disease and organ
transplantation. §
It includes lymphoid organs, tissues, cells & antibodies. |
LYMPHOID
ORGANS
These
are the organs where origin/maturation & proliferation of lymphocytes occur.
2 types: Primary & Secondary.
a. Primary
lymphoid organs
The organs where lymphocytes are matured &
differentiated to antigen-sensitive lymphocytes. It is 2 types:
1. Bone
marrow: The site of formation of all blood cells
including B & T-lymphocytes.
2. Thymus:
A bilobed organ seen near the heart and beneath the breastbone. It is large
during birth but gradually reduces in size and becomes very small size in
puberty. Immature T-lymphocytes from bone marrow is migrated to thymus and
matured.
b. Secondary
lymphoid organs
-
The organs, to which matured
lymphocytes migrate from primary lymphoid organs, interact with antigens and
then proliferate to become effector cells.
E.g. Spleen, lymph nodes, tonsils, Peyer’s patches, Mucosa-
associated lymphoid tissue (MALT) &
appendix.
- Spleen:
Bean-shaped organ. Contains lymphocytes and phagocytes. It removes worn-out RBCs & microorganisms from blood. It is a
reservoir of erythrocytes in foetus.
- Lymph nodes:
Found in lymphatic system. They trap microorganisms or other antigens. Trapped
antigens activate lymphocytes and cause immune response.
- MALT:
Located within the lining of respiratory, digestive &
urinogenital tracts. It constitutes 50% of lymphoid tissue.
IMMUNITY
It is the ability of the immune
system to fight the pathogens.
It is 2 types: Innate and
Acquired.
1. Innate (inborn) immunity
-
It
is the non-specific immunity present at the
time of birth.
-
It includes 4 types of Barriers:
a. Physical
barriers: Prevents entry of microbes. E.g. Skin, Mucus coating of the respiratory,
gastro-intestinal and urino-genital tracts. Mucus traps microbes.
b. Physiological barriers: They
prevent microbial growth. E.g. gastric HCl, saliva, tear etc.
c.
Cellular barriers: Phagocytes like WBC [Polymorpho-nuclear leukocytes (PMNL) or
neutrophils, monocytes and natural
killer lymphocytes], macrophages etc.
d. Cytokine barriers: Virus
infected cells secrete a cytokine protein called interferon. It protects
non-infected cells from further viral infection.
2. Acquired
(adaptive) immunity
· It is
pathogen specific immunity developed
during lifetime.
· It
is characterized by memory, i.e. during
first encounter of a pathogen, body produces primary response in low intensity. Second encounter of the same
pathogen causes a secondary (anamnestic)
response in high intensity.
· Primary
and secondary immune responses are carried out with B-lymphocytes (B-cells) and T-lymphocytes (T-cells).
a. B-lymphocytes:
Produce antibodies. These are the
proteins to fight the pathogens.
b.
T-lymphocytes:
Help B-cells to produce antibodies.
 Structure of an antibody molecule Types of antibodies: IgG, IgA, IgM, IgE & IgD. |
Types of Acquired
immune response
1.
Humoral
immune response/ Antibody mediated immunity (AMI): It is the immune response mediated by antibodies. Antibodies
are found in blood plasma. So called as Humoral immune response.
2.
Cell-mediated response / cell-mediated immunity (CMI): It is the immune response mediated by
T-lymphocytes (T-cells). The body can differentiate ‘self’ and
‘non-self’ and the CMI causes Graft rejection.
Tissue
matching & blood group matching are essential before undertaking any graft/
transplant. After this, the patient should take immuno-suppressants all his
life.
Types of Acquired immunity
Acquired immunity is 2
types: Active and passive.
1. Active immunity: It is the immunity in
which antibodies are produced in a host body when the host is exposed to antigens (e.g. living or dead microbes
or other proteins).
It
is a slow process. It is produced by 2 ways:
a. Natural
Active Immunity: It is developed during natural
infection by microbes.
b. Artificial Active Immunity: It
is developed by injecting the microbes deliberately
during immunization.
2. Passive immunity: Here, readymade antibodies are
directly given to the body. It is 2 types:
a. Natural
Passive Immunity: E.g.
§
Antibodies (IgG) from
mother → Placenta → Foetus
§
Antibodies (IgA) in
colostrum → infants
b. Artificial
Passive Immunity: E.g.
§
Anti-tetanus serum (ATS)
Immunization
This is based on ‘memory’
of the immune system. 2 types:
1. Active Immunization (Vaccination)
§ In
this, a preparation of vaccine (antigenic proteins of pathogen or
inactivated pathogen) is introduced into the body. It results in the
development of antibodies.
§
During actual infection,
the antibodies neutralize antigens.
§ The
vaccines also generate memory B and T-cells. They recognize the pathogen
quickly.
§ E.g.
Polio vaccine, Hepatitis B vaccine, DPT vaccine etc.
§ Vaccines
are produced using DNA recombinant technology (E.g. Hepatitis B vaccine
produced from Yeast).
2. Passive Immunization
§ It
is the direct injection of pre-formed antibodies or antitoxin. It is
required for quick immune response.
§ E.g. Immunization against Tetanus, snake venom etc.