Immunity

Definition of Immunity

Immunity refers to the resistance acquired by a host against injury caused by microorganisms and their products. It provides protection against infectious diseases and involves the body's reaction to foreign antigens.

Types of Immunity

1. Innate Immunity

• Definition: Resistance to infection that is present from birth due to genetic and constitutional makeup. It is not influenced by prior exposure to microorganisms or immunization.

• Characteristics:

Immediate response.
Non-specific defense.
No memory of previous infections.

Subtypes of Innate Immunity

1. Species Immunity: Resistance common to all members of a species (e.g., humans are resistant to Bacillus anthracis, which infects chickens).

2. Racial Immunity: Differences in susceptibility among races (e.g., genetic resistance to Plasmodium falciparum malaria in African populations).

3. Individual Immunity: Variations in immunity among individuals of the same species (e.g., homozygous twins show similar resistance to tuberculosis).

Factors Influencing Innate Immunity

Age: Immature immune system in fetuses; waning immunity in old age.
Hormonal Factors: Conditions like diabetes and adrenal dysfunction increase susceptibility.
Nutrition: Malnutrition weakens both humoral and cell-mediated immune responses.

Mechanisms of Innate Immunity

1. Epithelial Barriers:

Skin acts as a mechanical barrier and secretes bactericidal substances.
Mucosal surfaces trap pathogens (e.g., respiratory tract cilia propel particles out).
Gastric acid destroys bacteria in the stomach.

Antibacterial Substances:

• Proteins like lysozyme, complement, and interferons destroy pathogens.

1. Cellular Factors:

   • Phagocytes (e.g., neutrophils and macrophages) engulf and destroy pathogens through chemotaxis, attachment, ingestion, and intracellular killing.

2. Inflammation:

   • A nonspecific response to tissue injury that involves vasodilation, increased vascular permeability, and phagocytosis.

3. Fever:

   • Elevated body temperature inhibits pathogen growth and stimulates interferon production.

4. Acute Phase Proteins:

   • Proteins like C-reactive protein (CRP) activate the complement system to prevent tissue injury.

2. Adaptive Immunity

• Definition: A specific immune response that develops after exposure to antigens; it has memory, providing long-term protection.

• Characteristics:

Slower onset compared to innate immunity.
Highly specific to the antigen.
Involves immunological memory.

Types of Adaptive Immunity

1. Active Immunity:

   • Induced by infection or vaccination.

   • Long-lasting protection.

   • Examples:

     • Naturally Acquired: Infection with a live pathogen.

     • Artificially Acquired: Vaccination (e.g., BCG vaccine for tuberculosis).

2. Passive Immunity:

   • Provided by ready-made antibodies.

   • Short-term protection.

   • Examples:

     • Naturally Acquired: Antibodies passed from mother to fetus via placenta.

     • Artificially Acquired: Injection of antibodies (e.g., gamma globulin).

Mechanisms of Adaptive Immunity

1. Humoral Immunity:

Mediated by antibodies produced by B cells.
Neutralizes toxins, lyses antigens, or facilitates phagocytosis.

1. Cell-Mediated Immunity:

Mediated by T lymphocytes.
Effective against chronic bacterial infections and intracellular pathogens like viruses.

Antigens

• Molecules that induce an immune response in the body.

• Types:

  1. Exogenous Antigens: Enter from outside the body (e.g., inhalation or injection).

  2. Endogenous Antigens: Generated within cells due to infection or metabolism.

  3. Autoantigens: Normal proteins targeted in autoimmune diseases.

Key Terms

• Immunogen: A substance capable of inducing an immune response.

• Hapten: A small molecule that elicits an immune response only when attached to a larger carrier molecule.

• Epitope: The specific part of an antigen recognized by an antibody.

Antibodies (Immunoglobulins)

• Glycoproteins produced by plasma cells in response to antigens.

• Structure:

  • Composed of four polypeptide chains (two heavy and two light chains).

  • Variable regions bind specifically to antigens.

  • Constant regions determine antibody function.

Types of Antibodies

Key Differences Between Active and Passive Immunity