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‘Immunitas’- Latin;  ‘Freedom From’

Body’s ability to resist or eliminate potentially harmful foreign materials or abnormal cells.

Key Role of Immune Defense System:

n      To recognize and differentiate between ‘normal self’ and ‘non self’.

n      To destroy or neutralize harmful materials within the body

Significance & Functions:

  • Defense against invading pathogens; Bacteria & Viruses are the major targets.
  • Removal of worn- out (aged/senile) cells & debris.
  • Identification & destruction of abnormal or mutant cells i.e. ‘immune surveillance’.
  • Inappropriate immune responses Allergy & Autoimmune Diseases.
  • Rejection of tissue cells of foreign origin.

(The major obstacle in organ transplantation)

Life Span/Kinetics of Lymphocytes

n      Lymphocytes (B & T):

§          B-Lymphocytes —- “Bursa Fabricius”,  “Bone marrow”.

§         T-Lymphocytes —- “Thymus”.

    • Life span 100-300 days.
    • During this period majority of Lymphocytes continuously recycle among lymphoid tissues —- Lymph and back, spending only a few hours at a time in blood.


n      Can elicit specific immune response. “Antibody Generation”

n      Specific molecular group(s) present in their structure (‘Antigen determinant’ or ‘Epitope’)

n      Large polysaccharides or proteins

n      Mol wt  > 8000


n      Not Antigenic, Not protein

n      Mol Wt  <  8000

n      Drugs, Dust chemicals, Animal dandruff, Toxins, Industrial chemicals

n      On combination with proteins can elicit immune response

Immune Response:

Production of specific antibodies or T- Lymphocytes

on sensitization of immune system by an antigen.

Types of Immunity

Two types:

I. Natural or Innate immunity or Non-specific immunity:

  • Immunity due to General Physiological Mechanisms
  • Natural resistance against injury
  • Does not require previous exposure to antigen

II Acquired or Adaptive immunity or Specific immunity:

  • Not present at birth.
  • Develops after exposure to the antigen.
  • Specific or selectively targeted against particular foreign material.

Two Types:

n      Humoral or B-Cell immunity   (Antibodies)

n      Cell Mediated or T-cell immunity  (T-lymphocytes)

Natural or Innate Immunity (Non-Specific Immunity).

Immunity is provided by general processes like;

1.   Phagocytosis by WBCs & Monocyte – Macrophage system.

2.   Barriers against infections: Cilia, Mucus, Acid in stomach,

Digestive enzymes, Sebaceous secretions, Skin barrier.

3.   Inflammation.

4.   Nk – cells or Natural Killer Lymphocytes:

v        Non specifically destroy viral infected or cancer cells.

5.   Presence of Chemicals in blood e.g.

i.   Lysozymes, Mucolytic Polysaccharides destroy bacteria.

ii.  Basic Polypeptides: Inactivate certain Gram +ve bacteria.

iii. Complement complex.

iv. Interferon

2. Specific Immunity

I.   Acquired or Adaptive or Active Immunity

Ability to develop extremely powerful specific immunity against individual invading agents by forming Antibodies  and/or activated T-Lymphocytes that attack and destroy the specific invading organism or toxin.

a.  Humoral or B-Cell Immunity

“B-Lymphocytes”, Plasma cells, Antibodies

b.  Cell-Mediated Immunity:

“Activated T-Lymphocytes

II.  Passive Immunity

Injecting antibodies or activated T cells.

Role of Thymus:

Ø      Thymic Education

Ø      Release of Thymin (Maintains T-cell lineage by increased proliferation of new

T-cells & augmented activity of T-cells).

B-Cell Immunity

  • Antibodies (Immunoglobulins)
  • Types:

*  IgG

*  IgM

*  IgA

*  IgE

*  IgD

  • Vaccination

Mechanism of Action of Antibodies

Act mainly in two ways:

1. Direct Action on Invading Agents:

This mechanism is not very strong:-

    • Agglutination
    • Precipitation
    • Neutralization
    • Lysis

2. Activation of Complement system

Classical Pathway

Alternate Pathway

  • “Complement” is a system of 20 proteins which are enzyme precursors, normally present among plasma proteins.
  • The principal actor of the system are 11 proteins i.e. C1 to C9, B & D.


  • Opsonizaion  & Phagocytosis
  • Lysis
  • Agglutination
  • Neutralization of Viruses
  • Chemotaxis
  • Activation of Mast cells & Basophils
  • Inflammatory effects

Cellular Immunity

  • By Activated T-Lymphocytes
  • Specific for specific antigen
  • Receptor sites on T-cells (Approx 10,000)
  • Types of T-cells:

1. Helper T-cells

2. Cytotoxic T-cells

3. Suppressor T-cells

Helper (Inducer) T-cells

  • Most numerous (60-80% of total T-cells)
  • Help in the functions of immune system by secreting Lymphokines (IL-2, IL-3, IL-4, IL-5, IL-6. GM-CSF, Interferon-α)
  • Absence of Lymphokines from Helper T-cells→ Immune system almost paralyzed

e.g.  AIDS (Acquired Immune Deficiency Syndrome)


1. Stimulates;

  • Growth & Proliferation of Cytotoxic & Suppressor T-cells
  • Growth of B-cells & differentiation to plasma cells & Antibodies

IL- 4,5,6 (B-cell growth factors)

2.   Activation of Macrophage – system

By releasing ‘Macrophage Migration Inhibition’ factors

    Increased efficient Phagocytosis

    3.   Positive feed back stimulation of Helper-T-cells

    IL-2 acts as amplifier of Helper-T cell response

      Cytotoxic (Killer) T-cells

      • Attack cancer cells & Microorganisms and destroy them
      • Tissue cells invaded by Viruses
      • Body’s own cells (Autoimmune disease)
      • Activity is controlled by Helper T-cells

      Mode of Action:

        • Binding the cell
        • Releasing Cytotoxic substances directly into the cells
        • Secrete ‘Perforines’ (Hole forming proteins)

      Suppressor (Regulatory) T-cells

      • Suppress the functions of both Helper & Killer T-cells
        • Control excessive immune Reaction
        • Immune tolerance

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