Immunology Overview and Vaccination History

Early Vaccination Studies Led the Way to Immunology

• Louis Pasteur proved that ageing weakened the virulence of a fowl cholera bacterium.
• He used this discovery to develop a vaccine (from Latin "vacca" meaning cow) in honor of Jenner's work with cowpox.
• Pasteur successfully created a rabies vaccine for a young boy who had been bitten by a rabid dog.
• Joseph Meister became a caretaker at the Pasteur institute, which was established in 1887 to treat rabies patients.

Immunology Is About More than Just Vaccines and Infectious Disease

• Many infections are prevented by clean water, good hygiene, and nutrient-rich diets.
• Some infections are self-limiting and treatable, and are unlikely targets for expensive vaccination programs, including the common cold and cold sores.
• Some infectious agents are not suitable for vaccination due to a range of factors, including the number of molecular variants, the complexity of generating immunity, or the body's inability to establish adequate immune memory responses.

The Immune Response Is Composed of Two Interconnected Arms - Innate and Adaptive Immunity

• Innate immunity comprises evolutionarily primitive mechanisms preventing infection or quickly eliminating common invaders.
• It includes physical barriers and DNA-encoded receptors, recognizing common structures of many pathogens.
• Adaptive immunity is more attuned to subtle molecular differences and relies on B and T lymphocytes.
• It takes longer than innate immunity to mount an immune response but is more specific.

The Immune Response Is Composed of Two Interconnected Arms - Innate Immunity

• Innate defenses include physical and chemical barriers, as well as DNA-encoded receptors recognizing common pathogen structures.
• It also includes complement - a series of pre-existing serum proteins that initiate a cascade of labeling and destruction events against pathogens.

The Immune Response Is Composed of Two Interconnected Arms - Adaptive Immunity

• Adaptive immunity is slower than innate immunity because fewer cells possess the perfect receptor for the specific antigen and because it relies on prior encounter and categorization of antigens.
• T-cell and B-cell lymphocytes undergo selection and proliferation after an antigen encounter.
• Adaptive immunity evolves in response to infection and displays four key attributes:
  • Antigenic specificity. It can distinguish between even subtle differences among antigens.
  • Diversity. It can recognize billions of unique structures on foreign antigens.
  • Memory. It can confer lifelong immunity to many infectious agents after an initial encounter.
  • Self/nonself recognition. It responds only to foreign antigens, preventing inappropriate responses to self molecules.

Adaptive Immunity – Primary & Secondary Response

• Primary immune response occurs when an antigen is introduced for the first time in a host and results in the activation of T and B cells.
• The response produces memory cells and plasma cells.
• Plasma cells produce antibodies, while memory cells are long-lived and remain in the blood for a long time.
• The duration from antigen entry to the appearance of the first antibody in the blood is called the lag phase, which is normally 4-7 days.
• Secondary immune response occurs when the same antigen is injected into the host for the second time.
• Memory cells directly get converted into plasma cells when activated by the antigen, resulting in a shorter lag phase (1-3 days) and 100 times more antibody production.
• IgG is produced instead of IgM in secondary response, and the antibodies display a greater affinity for the antigen.

Adaptive Immune response – Humoral & Cellular

• Humoral immunity is mediated by macromolecules like antibodies, complement proteins, and antimicrobial peptides located in extracellular fluids.
• It contrasts with cell-mediated immunity and is associated with antibody production, cytokine production, germinal center formation, isotype switching, affinity maturation, and memory cell generation.
• Its effector functions include pathogen and toxin neutralization, classical complement activation, and opsonin promotion of phagocytosis.

Cellular Immunity

• Cellular immunity is an immune response that doesn't involve antibodies, instead relying on the activation of phagocytes, cytotoxic T-lymphocytes, and the release of various cytokines.
• It protects the body through:
  • T-cell mediated immunity: activating antigen-specific cytotoxic T cells that induce apoptosis in virus-infected cells or cancer cells.
  • Macrophage and natural killer cell actions: destroying pathogens through recognition, cytotoxic granules, and phagocytosis.
  • Cytokine secretion: influencing the function of other cells involved in immune responses.