Immune System

Key Concepts

• Pathogens: Disease-causing agents (viruses, bacteria, fungi, protists, parasitic worms).
• Lines of Defense: The body's layered protection against pathogens (first, second, and third).
• Nonspecific Defense: Immune responses that are not targeted to a specific pathogen (e.g., skin, inflammatory response).
• Specific Defense (Adaptive Immunity): Immune responses targeted to a specific antigen (cell-mediated and humoral responses).
• Antigen: A substance recognized by the body as foreign, often part of a pathogen.
• Cell-Mediated Response: An adaptive immune response involving cytotoxic T cells that destroy infected cells.
• Humoral Response: An adaptive immune response involving B cells that produce antibodies.
• Cytotoxic T Cell: A type of white blood cell that kills infected cells by inducing apoptosis.
• T-Helper Cell: A type of white blood cell that helps activate both cytotoxic T cells and B cells.
• B Cell: A type of white blood cell that produces antibodies.
• Antibody: A protein that binds to a specific antigen, deactivating the pathogen or marking it for destruction.
• Memory Cells (B and T): Long-lived immune cells that "remember" specific antigens, allowing for a faster response upon re-exposure.
• Vaccines: Introduce weakened or inactivated pathogens to stimulate an immune response and create memory cells.
• Complement System: A system of proteins that enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and attack the pathogen's cell membrane.
• Histamine: A compound released by mast cells that causes blood vessels to dilate and become leakier.
• Macrophages: A type of white blood cell that engulfs and digests pathogens and cellular debris.
• Apoptosis: Programmed cell death.
• Perforin: A protein released by cytotoxic T cells that creates pores in the cell membrane of infected cells.

Lines of Defense

The immune system protects the body from pathogens through three lines of defense:

1. First Line of Defense: External barriers like skin and mucous membranes prevent pathogen entry. This defense is nonspecific, blocking all types of pathogens.
2. Second Line of Defense: The inflammatory response is triggered when pathogens breach the first line. For example, a splinter introduces bacteria, causing mast cells to release histamine. Histamine dilates blood vessels and increases their permeability, allowing macrophages to reach the site and engulf pathogens. The complement system also enhances the immune response by attracting more macrophages. This defense is also nonspecific.
3. Third Line of Defense: Adaptive immunity, a specific response to antigens, is activated when the first two lines are insufficient. This involves cell-mediated and humoral responses.

Cell-Mediated Response (Specific)

The cell-mediated response involves cytotoxic T cells, which destroy infected cells.

• Mechanism: Cytotoxic T cells induce apoptosis (self-destruction) in infected cells by releasing perforin, which creates holes in the cell membrane, causing the cell to lyse.
• Activation:
  • Infected cells present antigens on their surface, signaling to cytotoxic T cells.
  • Macrophages consume pathogens, process them, and present antigens on their surface. T-helper cells bind to these macrophages and release chemical signals that stimulate cytotoxic T cells.
• Function: Cytotoxic T cells target and destroy infected cells, preventing pathogen replication and amplifying the immune response.

Humoral Response (Specific)

The humoral response involves B cells, which produce antibodies.

• Mechanism: B cells produce antibodies, which are Y-shaped proteins that bind to specific antigens.
• Activation:
  • Macrophages present antigens to T-helper cells, which then stimulate B cells.
  • B cells can also be activated directly by free antigens.
• Antibody Function:
  • Deactivate pathogens by interfering with their movement, reproduction, or ability to cause harm.
  • Mark pathogens for destruction by macrophages.
• Antibody Classes: Different classes of antibodies exist, such as IgE, which protects against parasitic worms and is involved in allergic reactions.

Memory Cells and Vaccines

Both cell-mediated and humoral responses generate memory cells (Memory B cells and Memory T cells).

• Memory Cells: These cells "remember" specific antigens, allowing for a faster and more efficient immune response upon re-exposure. Memory B cells activate Plasma B cells which will make antibodies. Memory T cells activate cytotoxic T cells which will go after infected cells.
• Vaccines: Vaccines introduce weakened or inactivated pathogens, triggering an immune response without causing disease. This leads to the formation of memory cells, providing long-term immunity.

Conclusion

The immune system is a complex and multifaceted system that protects the body from pathogens through a layered defense strategy. The nonspecific defenses (first and second lines) provide immediate protection, while the specific defenses (cell-mediated and humoral responses) target specific antigens and create long-term immunity through memory cells. Vaccines leverage this system to provide protection against specific diseases.