Immune System Responses: Cells, Clonal Selection, and Immunity Types

T-Cell Coreceptor Specificity

• Helper T-cells require antigen and Class II markers.
• Co-presentation requirements stem from different coreceptors on Killer and Helper T cells:
  • Killer T-cell coreceptor CD8 interacts only with Class I markers.
  • Helper T-cell coreceptor CD4 interacts only with Class II markers.

Interactions: Antigen-Presenting Cells & Lymphocytes

Active Immunity

Primary and Secondary Immune Responses

1. On first exposure to a pathogen, there is a latency of 5-10 days before specific antibodies are made, known as the primary response.
   • Antibody levels plateau after a few days and decline after a few weeks.
2. Subsequent exposure to the same antigen causes a secondary response.
   • Antibody production is much more rapid and sustained.

Clonal Selection Theory

Mechanism for Secondary Immune Responses

1. Each B cell produces only one kind of antibody and a related antigen receptor on its surface.
2. Exposure to its antigen stimulates a B cell to divide until a large population of genetically identical cells (clones) is produced.
   • Some of these become plasma cells and secrete antibodies.
   • Some become memory cells that can be stimulated to produce antibodies in the secondary response.

Clonal Deletion

• Immune tolerance of self-antigens.
• Prevents autoimmunity (attack on self-tissue by the immune system).
• Removal of B and T cells that recognize "self-antigens."

Types of Immunity

Herd Immunity

Immunity to a pathogen by most members of a population means that the population is not a reservoir for the pathogen, providing protection for susceptible individuals (e.g., children, elderly, sick, immunocompromised).

Stimulation of Active Immunity

1. Naturally Acquired Active Immunity

   Exposure to antigens results in illness and recovery, often leading to lifetime immunity for some diseases.

   • Development of a secondary response provides active immunity.

2. Artificially Acquired Active Immunity

   Immunizations induce primary responses by inoculating people with pathogens whose virulence has been attenuated or destroyed (vaccinations).