Adaptive Immunity and the Immune Response: Microbiology Study Notes

Chapter 15: Adaptive Immunity and the Immune Response

Glimpse of History

The study of adaptive immunity has its roots in the investigation of infectious diseases such as diphtheria, which historically killed many infants and small children. Early research led to the discovery of the mechanisms behind immune protection and the development of antitoxins.

• Diphtheria: Caused by club-shaped bacteria discovered by Frederick Loeffler.

• Toxin Hypothesis: Loeffler hypothesized that the bacteria produced a poison (toxin) that spread in the blood.

• Experimental Evidence: Emile Roux and Alexandre Yersin extracted the toxin and showed its fatal effects in guinea pigs.

• Antitoxin Discovery: Emil von Behring demonstrated that animals recovered from diphtheria were protected by something in their blood, termed antitoxin.

• Therapeutic Application: Antitoxin mixed with serum from immune animals protected others from diphtheria.

• Nobel Prizes: Emil von Behring (antitoxin therapy), Rodney Porter and Gerald Edelman (antibody structure).

Adaptive Immunity Develops Throughout Life

Adaptive immunity is the most effective means to eliminate invading microbes, but it requires time to develop after first exposure. Innate immunity provides initial protection during this period.

• Development: Takes a week or more to build after first exposure.

• Innate Immunity: Must protect during the initial phase.

• Memory: Adaptive immunity has memory, leading to a stronger response upon re-exposure.

• Vaccination: Relies on the memory feature of adaptive immunity.

• Molecular Specificity: Response is specific to particular antigens.

• Self vs. Non-self: Must distinguish between healthy self and dangerous entities (pathogens, cancerous cells).

Strategy of the Adaptive Immune Response

The adaptive immune system employs two main strategies to counter foreign materials, each targeting different types of antigens.

• Primary Response: The first response to an antigen; slower and less robust.

• Secondary Response: Stronger and faster upon re-exposure due to memory cells.

• Humoral Immunity: Eliminates extracellular antigens (bacteria, toxins, viruses in bloodstream and tissue fluids).

• Cell-mediated Immunity (CMI): Targets antigens residing within host cells (e.g., viruses infecting cells).

• Regulation: Both systems are powerful and tightly regulated to prevent damage to the body's own tissues.

Comparison of Humoral and Cell-Mediated Immunity

Feature	Humoral Immunity	Cell-Mediated Immunity
Target	Extracellular antigens	Intracellular antigens
Main Cells	B lymphocytes (B cells)	T lymphocytes (T cells)
Effector Molecules	Antibodies	Cytokines, cytotoxic molecules
Examples	Bacteria, toxins, viruses in fluids	Viruses inside cells, cancer cells

Key Terms and Concepts

• Antigen: A molecule that reacts specifically with an antibody, B-cell receptor, or T-cell receptor.

• Antitoxin: A substance in blood that neutralizes toxins.

• Memory Cells: Long-lived cells that enable a rapid and strong secondary immune response.

• Vaccination: The process of inducing immunity by exposure to antigens in a controlled manner.

• Self vs. Non-self Recognition: The immune system's ability to distinguish between the body's own cells and foreign invaders.

Example: Diphtheria Immunity

Children historically died from diphtheria until the discovery of antitoxins and the development of vaccines. The adaptive immune response, specifically the production of antibodies, is crucial for protection against the diphtheria toxin.

Additional info: The adaptive immune system's ability to remember and respond more effectively to previously encountered antigens forms the basis for modern vaccination strategies and immunological research.