Immunity

‘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.

Antigen

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

Haptens

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.

Effects

• 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)

Functions:

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