‘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
Production of specific antibodies or T- Lymphocytes
on sensitization of immune system by an antigen.
Types of Immunity
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.
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.
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.
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:
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).
- Antibodies (Immunoglobulins)
Mechanism of Action of Antibodies
Act mainly in two ways:
1. Direct Action on Invading Agents:
This mechanism is not very strong:-
2. Activation of Complement system
- “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
- Neutralization of Viruses
- Activation of Mast cells & Basophils
- Inflammatory effects
- 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)
- 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