Here we will learn about the immune system, which combats disease-causing organisms.
Pathogens
Disease-causing or harmful microorganisms
Antigens
Material that can evoke an immune response
Non-specific, fast
The
innate branch is non-specific (it reacts to a broad range of microbes) and is fast (responses occur within hours of infection).
Actors of the Innate System
Physical barriers:
- Epidermal cells create a slightly acidic surface and release enzymes and other antimicrobial peptides that make the skin surface inhospitable to microbes.
Chemical barriers:
- Some body surfaces (such as the respiratory pathways) are also covered in mucus, which can trap microbes before they can infect the body.
Chemokines:
Chemical signal produced by damaged cell to alert the body to danger and act as a homing signal for immune cells
Neutrophils:
First type of phagocytic cell to arrive
Monocytes:
Arrive and mature into macrophages which engulf and destroy pathogens
Inflammation:
Response to tissue damage, four clinical signs: redness, heat, swelling and pain.
Innate System in Action
Pathogens gain entry to the inside of the body through the wound.
Chemokine chemical signals (chemotactic cytokines):
Released by damaged cells to alert the body of trouble.
Act as homing signals that stimulate cells of the immune system to migrate towards the source of the chemokines.
Phagocytic cells are then attracted to the wound site from the blood.
Neutrophils, monocytes, macrophages.
Macrophages phagocytose pathogens:
Phagocytosis is a type of bulk transport into the cell in which large extracellular cargo is brought into the cell and broken down.
In our example, the bacterium will eventually be broken down and the macrophage will go on to engulf and destroy other pathogens.
Clinical signs of inflammation
Result from innate immunity activation.
Redness, heat, swelling, and pain are all classically accepted signs of inflammation.
Importantly, inflammation can result in impairment in function.
If the innate branch is unable to fight off the invading pathogens on its own, the adaptive branch is called in to help.
Specific, slow, systemic, memory
The adaptive branch is:
Specific (it can distinguish specific species of pathogens)
Slow (responses occur within days of infection).
Systemic (NOT restricted to the initial site of infection).
Has memory (it mounts a faster and even stronger attack against repeat pathogens).
Actors of the Adaptive System
Adaptive immunity is further divided into humoral immunity and cell-mediated immunity.
Humoral immunity: B cells (matured into plasma cells) producing antibodies (Y-shaped proteins)
Cell-mediated immunity: Cytotoxic T cells recognize infected cells and kill them while helper T cells act as the general of the immune army and release chemical signals that activate various immune cell types
Lymphoid tissue is the home of many types of immune cells.
Plasma cells reside in the lymphoid tissues:
- Plasma cells are activated and matured B cells (B cells and T cells are the two adaptive immune cell types).
- Plasma cells pump out antibodies.
Antibodies are Y-shaped proteins that recognize antigens from the invading pathogen.
The antibodies produced by plasma cells are released into the blood vessel where they travel throughout the body.
In the tissues, we show an antibody binding two viruses.
Neutralizing antibodies are able to bind to viruses and toxins in such a way as they are no longer able to infect or cause damage.
The major cells of cell-mediated immunity are the T cells, which are divided into cytotoxic T cells or helper T cells.
Cytotoxic T cells have receptors on their surfaces that bind infection signals on infected host cells.
The cytotoxic T cell has a receptor on its surface that is able to bind the infected cell's signal.
When this happens, the T cell releases death signals which kill the infected cell.
Helper T cells act as the general of the immune army.
They release activation signals (cytokines and chemokines) that help phagocytic cells to perform their job more efficiently.
They also release signals that activate cytotoxic T cells.
B cells also receive activation signals from helper T cells.
With all of these cells working together, the immune system is usually able to destroy the invading pathogens.
