innate immunity part 2 video 1

Overview of the Immune System

The immune system consists of two main responses: innate and adaptive.
The innate immune response is the first line of defense against pathogens and is quicker than the adaptive immune response.
Two potential outcomes of the immune response:
- Infection is cleared.
- Host damage occurs due to collateral damage from the immune response.

Inflammatory Response:
- The innate response can sometimes aid in spreading infections (e.g., colds) leading to increased symptoms like swelling and mucus production.
- Not always caused by the pathogen but can be a reaction from the immune system.
- Example: In Schistosomiasis, the inflammatory response helps the parasite expel its eggs through the blood vessel walls.
Consequences of the Inflammatory Response:
- Can promote pathogen spread, as seen with the common cold.
- The body’s reaction can sometimes be more damaging than the pathogen itself.

The body detects foreign invaders through Pattern Recognition Receptors (PRRs).
- PRRs recognize microbial patterns or host cell damage.
Phagocytosis:
- Phagocytic cells have PRRs that bind to pathogens or debris, stimulating phagocytosis, which eliminates invading organisms and damaged cells.

Introduction to Complement:
- Complement is a group of 20 proteins in the bloodstream and tissue fluids that function as both sensors and effectors of the immune system.
- Proteins exist in inactive forms until activated by microbial invaders, triggering an enzymatic cascade.
Complement Activation Cascade:
- Activated by foreign antigens, typically recognized by antibodies, which cleave complement proteins.
- Key enzyme: C3 convertase
  - Converts inactive C3 into C3a and C3b.
  - Both alpha and beta fragments play critical roles in immune response.

Three Main Consequences:
- Cell Lysis:
  - C5b initiates the formation of the membrane attack complex (MAC), causing cell lysis in pathogens.
- Opsonization:
  - C3b coats microbes, enhancing phagocytosis by allowing phagocytes to easily attach to pathogens (like velcro).
  - Example: Macrophages use extendable membranes to engulf bacteria.
- Enhanced Inflammatory Response:
  - C3a and C5a promote vascular permeability, allowing immune cells to exit blood vessels and reach the site of infection.

Three Activation Pathways:
- Lectin Pathway: Triggered by mannose-binding lectin (MBL) that recognizes bacterial carbohydrate patterns.
- Classical Pathway: Initiated by antibodies binding to antigens on pathogens.
- Alternative Pathway: Begins when C3b binds to microbial surfaces, prompting further action in complement activation.

The immune system must regulate the complement system to avoid damage to its own cells.
Decay Accelerating Factor (DAF):
- A protein that disrupts the complement activation on host cells to prevent lysis.
Example of Disease:
- Paroxysmal nocturnal hemoglobinuria, a rare condition resulting from a lack of DAF, leading to unregulated complement activation and lysis of red blood cells.

Complement proteins form a crucial part of the immune defense, activated through various pathways with prominent consequences including lysis, opsonization, and amplification of inflammation.
Pathogens have ways to evade complement recognition, and the host must maintain balance to prevent self-damage.