17. IMMUNOLOGY

Introduction

Immunology is the study of the body’s defence system. It explains how our body identifies foreign substances, produces antibodies and protects itself from infections. Anything that is foreign to the body is called an antigen. When an antigen enters the body, the immune system produces antibodies to neutralize it.

Immunity

Immunity is the ability of the body to fight microorganisms and their toxins. The immune system differentiates “self” from “non-self” and responds to harmful foreign substances. Pathogens cause disease, and their ability to do so is called virulence. Weakening the virulence creates an attenuated organism used in vaccines.

Types of Immunity

Immunity is broadly divided into innate immunity and adaptive immunity.

1. Innate Immunity

Innate immunity is the body’s natural, non-specific defence present from birth. It includes:

  • External barriers: skin, mucous membranes, saliva, tears, cilia.
  • Internal mechanisms: phagocytes, natural killer cells, complement proteins, interferons, fever response.

Phagocytosis

Phagocytosis is the process by which phagocytic cells like macrophages and neutrophils engulf and destroy microbes. The antigen is enclosed in a phagosome which fuses with a lysosome to form a phagolysosome. Enzymes and chemicals then kill the pathogen.

Steps in Phagocytosis

  • Recognition and attachment of antigen
  • Engulfment by pseudopodia
  • Formation of phagolysosome
  • Digestion and destruction of the microbe

2. Adaptive (Acquired) Immunity

Adaptive immunity develops after exposure to an antigen. It is specific and has memory. It has two components:

  • Humoral immunity: Involves B cells and antibodies (immunoglobulins).
  • Cell-mediated immunity: Involves T cells such as helper T cells and cytotoxic T cells.

B cells produce antibodies that circulate in the blood, while T cells destroy infected cells. Antigen-presenting cells like macrophages activate T cells through cytokines and interleukins.

Acquired Immunity: Active & Passive

Acquired immunity can be:

1. Naturally Acquired Immunity

  • Active: After infection, the body produces antibodies.
  • Passive: Antibodies received from mother through placenta or breast milk.

2. Artificially Acquired Immunity

  • Active: Through vaccines, which stimulate antibody formation.
  • Passive: Through injection of ready-made antibodies (antisera).

Antigens

An antigen is any substance that triggers antibody production. Antigens have immunogenicity (ability to produce immune response) and reactivity (ability to react with antibodies). Small antigens that require carriers are called haptens.

Factors Affecting Antigenicity

  • Foreignness
  • Molecular size
  • Chemical complexity
  • Physical form
  • Degradability
  • Age and genetics
  • Route and dose of administration
  • Presence of adjuvants

Antibodies (Immunoglobulins)

Antibodies are Y-shaped proteins produced by B cells. Each antibody has two heavy chains and two light chains. The top ends form the variable region that binds to antigens.

Types of Immunoglobulins

  • IgG: Most abundant; crosses placenta; long-lasting protection.
  • IgM: First antibody produced; pentamer structure.
  • IgA: Found in mucosal secretions; protects respiratory and GI tracts.
  • IgD: Present on B-cell surface; role in activation.
  • IgE: Involved in allergies and parasitic infections.

Production of Antibodies

Steps include:

  • Antigen attaches to macrophage.
  • Activation of helper T cells.
  • Activation and differentiation of B cells.
  • Plasma cells produce antibodies.
  • Memory cells remain for long-term protection.

Antigen–Antibody Reactions

When an antibody binds to its specific antigen, a reaction occurs. Major reactions include:

  • Agglutination: Clumping of particles; used in blood grouping.
  • Precipitation: Soluble antigen forms insoluble complex.
  • Neutralization: Antibodies block toxins or viruses.
  • Complement fixation: Used for diagnosing infections.

Bacterial Toxins

Bacteria produce toxins that damage host cells. They are of two types:

1. Exotoxins

  • Released by bacteria (mostly Gram-positive).
  • Highly potent, heat labile, strongly antigenic.
  • Types: cytotoxins, neurotoxins, enterotoxins.
  • Can be converted to toxoids for vaccines.

2. Endotoxins

  • Part of Gram-negative bacterial cell wall.
  • Released on cell lysis.
  • Cause fever, shock, and inflammation.
  • Heat stable and weakly antigenic.

Vaccines

Vaccines stimulate immunity without causing disease. Types include:

  • Live attenuated vaccines: BCG, oral polio, measles.
  • Killed vaccines: typhoid, cholera, rabies.
  • Toxoid vaccines: diphtheria, tetanus.
  • Subunit vaccines: pneumococcal, meningococcal.
  • Recombinant vaccines: Hepatitis B.
  • DNA vaccines: HPV, influenza (newer).

National Immunization Schedule (Basic)

  • BCG – at birth
  • OPV – birth, 6, 10, 14 weeks, booster at 16–24 months
  • Hepatitis B – birth, 6, 10, 14 weeks
  • DPT – 6, 10, 14 weeks; boosters at 16–24 months and 5 years
  • Measles/MMR – 9–12 months and 16–24 months
  • TT – at 10 and 16 years

Importance of Booster Dose

Booster doses strengthen immunity when memory cells weaken. They help maintain long-term protection, especially against tetanus, diphtheria and polio.

Detailed Notes:

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