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Detailed Notes on Innate and Adaptive Immunity

Overview of the Immune System

The immune system comprises two main branches: the innate and adaptive immune systems. These systems work independently and collaboratively to recognize and combat microbes.

Innate Immune System

The innate immune system is characterized by its broad specificity and rapid action. It provides immediate defense against pathogens through:

  • Barrier Defenses: Physical barriers that prevent microbes from entering the body.
  • Internal Defenses: Pre-existing internal mechanisms that quickly eliminate microbes upon infection.

Adaptive Immune System

In contrast to the innate immune system, the adaptive immune system takes time to develop but offers highly specific and effective responses. Key features include:

  • Specificity: Ability to distinguish between different pathogens (e.g., SARS-CoV-2 and influenza virus).
  • Humoral Response: Involves the use of antibodies.
  • Cell-Mediated Response: Involves the use of T cells.

Phases of the Immune Response

The immune response can be broken down into sequential phases:

  1. Exposure to Pathogen: Initial contact with a pathogen.
  2. Anatomic Barrier: Physical barriers attempt to prevent entry.
  3. Nonspecific Effector Cells and Molecules: Broadly specific cells and molecules attempt to eliminate the pathogen.
  4. Induced Innate Immune Response: Activation of effector cells leading to an inflammatory response (minutes to hours).
  5. Adaptive Immune Response: Activation of B and T cells in lymphoid organs, resulting in a specialized response (hours to days).

Failure to clear the infectious agent can lead to severe disease or death.

Adaptive Immune Response in Detail

The adaptive immune response is initiated by the recognition of pathogens by naive T and B cells, which are initially few in number. The process involves:

  1. Recognition: Naive T and B cells recognize the pathogen.
  2. Expansion: Proliferation of cells to build an "army."
  3. Specialization: Differentiation into effector cells that eliminate microbes.
  4. Contraction: Reduction in the number of effector cells after the microbe is eliminated.
  5. Memory: Generation of memory cells for enhanced secondary responses.

Vaccination aims to generate memory cells for a quick response upon subsequent exposure.

Features of Innate and Adaptive Immunity

Speed of Response

  • Innate: Rapid response (minutes to hours).
  • Adaptive: Slower response (days to weeks).

Components

  • Innate: Physical barriers, pattern recognition receptors, phagocytes, and granulocytes.
  • Adaptive: T cells, B cells (lymphocytes), and antibodies.

Kinetics

Innate:

  1. Recognition of pathogen.
  2. Mounting an inflammatory response.
  3. Elimination of microbe (minutes to hours).

Requires only a single signal due to preformed cells and molecules.

Adaptive:

  1. Recognition of pathogen.
  2. Signals from the innate immune system.
  3. Activation of T and B cells.
  4. Migration to the site of infection.
  5. Clearance of infection (days to weeks).

Requires multiple signals and clonal expansion of activated cells.

Secondary Exposure

  • Innate: Responds the same way upon each exposure.
  • Adaptive: Generates memory cells, leading to a quicker, stronger, and longer-lasting response upon secondary exposure.

Mathematically, Figure 1 shows the magnitude of the immune response over time. The primary response magnitude for innate immunity can be described as I1(t). The secondary response magnitude is I2(t). Based on the transcript, I1(t) \approx I2(t).

However the primary response magnitude of adaptive immunity A1(t) is considerably less than the secondary response magnitude A2(t), that is A1(t) < A2(t). Also A2(t) occurs sooner than A1(t).

Properties of Adaptive Immunity

Specificity

  • Innate: Broadly specific; can differentiate between bacteria, fungi, and viruses.
  • Adaptive: Highly specific; can distinguish between different viruses (e.g., SARS-CoV-2, influenza virus, chickenpox virus).

Diversity

  • Innate: Limited diversity with a set number of receptors.
  • Adaptive: High diversity with billions of specificities, allowing recognition of almost anything.

Memory

  • Innate: No memory; responds the same way upon each exposure.
  • Adaptive: Has memory; generates a quicker response upon subsequent exposure (e.g., after vaccination).

Non-Reactivity to Self

  • Both innate and adaptive immune systems are non-reactive to self, preventing responses against the body's own cells.
  • Innate: Receptors recognize molecules found only on pathogens.
  • Adaptive: Cells undergo a training program to eliminate autoreactive cells.

Clonal Expansion

  • Innate: Does not clonally expand; instead, more cells are recruited from the bone marrow.
  • Adaptive: Undergoes clonal expansion, increasing the number of lymphocytes specific for a pathogen.

Specialization

  • Innate: General specialization (e.g., antiviral or antibacterial response).
  • Adaptive: Tailored responses against specific pathogens or toxins.

Contraction and Homeostasis

  • Both systems undergo contraction after eliminating the microbe to return to normal.
  • Adaptive: Effector cells die, leaving memory cells.