Immune System 1: Threat to The Body

Definition of Pathogens: Pathogens are organisms capable of causing disease in humans and other animals.
The Four Main Types of Pathogens:
- Bacteria: Many replicate in tissue areas outside of the host cells; however, some are capable of replicating inside host cells. Examples include:
  - Salmonella
  - Shigella
  - Staphylococcus aureus
- Viruses: These pathogens replicate strictly inside host cells. Examples include:
  - Influenza
  - Sars COVID-19
  - Hepatitis
- Fungi: These spread by producing spores. Infections occur when spores are inhaled or come into contact with a human. Examples include:
  - Candida albicans (Yeast)
  - Aspergillus flavus
  - Cryptococcus neoformans
- Parasites: Organisms that live on or in a host. Examples include:
  - Helminths (such as tapeworm and hookworm)
  - Plasmodium falciparum (the Malaria Parasite)

Pathogenesis: This refers to the specific process by which a disease or disorder develops. It encompasses various factors contributing to the onset of the disease, as well as its progression and long-term maintenance.
Pathogenicity: The inherent capacity of an organism to cause disease.
Host: A larger organism that provides an environment supporting the survival and growth of a smaller organism (the pathogen).
Pathogen (General): Any organism that causes an infectious disease.
Primary (Frank) Pathogen: An organism that causes disease through direct interaction with a healthy host.
Opportunistic Pathogen: An organism that may be part of the host's normal flora but causes disease when it gains access to unintended tissue sites or when the host becomes immunocompromised.
Infection: The condition in which a pathogen grows and multiplies within or on a host. An infection may or may not result in overt (visible) infectious disease.

To successfully cause disease and maintain its cycle, a pathogen generally follows these steps:

Maintain a Reservoir: The pathogen must have a habitat where it normally lives.
Leave Reservoir via Portal of Exit: The pathogen must leave its habitat to find a new host.
Adhere to the Surface of the Host: The pathogen must attach itself to the host's body.
Invade the Body of the Host: The pathogen enters the tissues or cells.
Evade the Body’s Defences: The pathogen must bypass or defeat the host's immune system.
Multiply within the Body: The pathogen replicates, often causing damage to host tissues.
Leave the Body and Return to Reservoir or Enter a New Host: The pathogen exits to continue the cycle of infection.

Reservoirs: The habitat in which an infectious agent normally lives, grows, and multiplies. It is important to note that the reservoir may or may not be the direct source from which an agent is transferred to a host.
- Environmental Reservoirs: Examples include plants, soil, and water. For instance, the reservoir for Clostridium botulinum is soil, but most botulism infections come from improperly canned food containing its spores.
- Human Reservoirs: Many diseases like Sexually Transmitted Diseases (STDs), measles, and mumps are hosted by humans.
- Animal Reservoirs: Some agents live in animals and can be transmitted to humans.
Portals of Exit: The specific path by which a pathogen leaves its host.
- Respiratory Tract: Influenza viruses, COVID-19, and Mycobacterium tuberculosis exit through coughing or sneezing.
- Skin/Blood: Hepatitis B can exit through cuts or needles in the skin.
- Vectors: Plasmodium (Malaria) exits when a mosquito bites an infected host.

Adhesion Strategies: Pathogens use adhesins to bind tightly to body surfaces. Adhesins are specialized proteins that bind specifically to receptors on host cells.
- Bacteria often produce multiple types of adhesins. This allows them to attach to different types of host cell receptors, helping them evade the immune system and attack various host cell types.
Invasins: These are a special class of adhesins that trigger host cells to take the pathogen in. They can effectively "trick" phagocytes into engulfing the pathogen.
Portals of Entry: The manner in which a pathogen enters a host. Pathogens often use the same portal to enter a host that they used to exit the previous host.
- Examples:
  - Respiratory Mucosa: Used by Influenza virus.
  - Gut Mucosa: Used by Salmonella.
  - Intact Skin: Hookworms can penetrate healthy skin.
  - Cuts and Broken Skin: Tetanus enters through wounds.
  - Mosquito Bites: Dengue virus and Plasmodium enter through the skin via vectors.
  - Intravenous (IV) Injection: HIV can be transmitted via needles.
  - Fecal-Oral Route: Pathogens exit via feces and enter a new host through the mouth (e.g., via unwashed hands or contaminated food).

Avoiding Phagocytosis:
- Capsule (Glycocalyx): A structure external to the bacterial cell wall that prevents direct contact with the host cell by covering the bacterial antigens.
- Surface Proteins: Specialized proteins that interfere with host cell contact, making it impossible for the host cell to reach the pathogen's antigens.
Active Evasion:
- Antigenic Variation: The pathogen periodically changes its surface antigens. By the time the immune system organizes a response, the antigen has changed and is no longer recognizable.
- $IgA$ Proteases: These are enzymes that destroy host antibodies. $IgA$ is a specific host antibody meant to prevent pathogens from attaching to host cells.
- Serum Resistance: A mechanism that protects bacteria from the complement system, which would otherwise create holes in the bacterial cells.

Process: Once inside, pathogens multiply and cause disease. They often kill host cells and tissues to make more nutrients available for their own growth.
Toxins: Pathogens produce poisonous products that damage human tissue.
- Exotoxins: Proteins secreted by the pathogen specifically to kill host cells.
- Endotoxins: These are the lipopolysaccharide ( $LPS$ ) components found on the outer membranes of gram-negative bacteria.
Spread: Pathogens can spread to other parts of the body via:
- Cell-to-cell contact
- The blood stream
- The lymphatic system
- The nervous system

Defense Mechanism: The immune system serves as a complex defense system to protect the host from threats.
Primary Objectives:
- Protect the host against invasion by pathogens.
- Fight and eliminate pathogens that have successfully invaded and infected the host.

Susceptible Hosts: While anyone can be infected, groups at higher risk include the elderly, infants, and the immunocompromised.
Common Portals of Entry/Exit:
- Entry: Mouth, Nose, Eyes, Cuts in skin.
- Exit: Coughing, Sneezing, Bodily Secretions, Feces.
Modes of Transmission:
- Direct Contact
- Indirect Contact
- Vectors (e.g., mosquitoes)