Threat to The Body

Introduction

  • The immune system is the body's defense mechanism aimed at eliminating pathogens that invade the host.

Pathogens

Overview

  • Pathogens are organisms that can cause disease in humans and other animals.

  • There are four main types of pathogens:   - Bacteria
      - Viruses
      - Fungi
      - Parasites

Types of Pathogens

Detailed Explanation of Pathogen Types

  1. Parasites    - Organisms that live on or in a host and get their food at the host's expense.    - Spread by producing spores that can be inhaled or come into contact with humans.    - Many replicate inside host cells, while some replicate outside of host cells.    - Examples:      - Helminths - (e.g., tapeworm, hookworm)      - Plasmodium falciparum - (the malaria parasite)

  2. Fungi    - Spread via spores.    - Examples:      - Candida albicans (Yeast)      - Aspergillus flavus      - Cryptococcus neoformans

  3. Viruses    - Require host cells to replicate.    - Examples:      - Influenza      - SARS      - COVID-19      - Hepatitis

  4. Bacteria    - Can replicate within host tissues.    - Examples:      - Salmonella      - Shigella      - Staphylococcus aureus

Pathogenesis

Definition and Key Terms

  • Pathogenesis:   - The process by which a disease or disorder develops, including factors contributing to its onset, progression, and maintenance.

  • Pathogenicity:   - The capacity of an organism to cause disease.

  • Host:   - The larger organism that supports the survival and growth of a smaller organism, such as a pathogen.

  • Pathogen:   - Any organism causing infectious disease.   - Two types:     - Primary (Frank) Pathogen:       - Causes disease by direct interaction with a healthy host.     - Opportunistic Pathogen:       - Part of normal flora; causes disease when it has access to different tissues or when the host is immunocompromised.

  • Infection:   - Occurs when a pathogen grows and multiplies within/on a host, potentially leading to disease.

Steps of Pathogenesis

  1. Maintain a Reservoir:    - The habitat where the infectious agent normally lives, grows, and multiplies.    - Example: The reservoir of Clostridium botulinum is soil, but infections can occur from improperly canned food containing its spores.

  2. Leave the Reservoir via Portal of Exit:    - Pathway for the pathogen to exit the host, e.g., respiratory tract, skin, feces.    - Examples:      - Influenza and COVID-19 exit via respiratory tract.      - Hepatitis B exits through skin cuts or needles.      - Plasmodium exits when mosquitos bite.

  3. Adhere to the Surface of the Host:    - Using adhesins, pathogens bind tightly to body surfaces, evading the immune system.    - Invasins are a special class of adhesins that trick host cells into internalizing the pathogen.

  4. Invade the Body of the Host:    - Pathways include:      - Upper respiratory tract
         - Lungs
         - Skin
         - Fecal-oral route from unwashed hands or contaminated food.    

  5. Evade the Body’s Defenses:    - Strategies include:      - Capsule (Glycocalyx):        - Covers antigens on bacterial walls to avoid detection.      - Surface Proteins:        - Interfere with host recognition.      - Antigenic Variation:        - Pathogen changes its antigens, evading immune response.      - IgA Proteases:        - Enzymes that destroy antibodies preventing pathogen attachment.      - Serum Resistance:        - Protection from the host's complement system, which can damage bacterial cells.

  6. Multiply within the Body:    - Pathogens grow and proliferate, killing host cells/tissues, which provides nutrients for themselves.    - Toxins:      - Exotoxins:        - Secreted proteins that kill host cells.      - Endotoxins:        - Lipopolysaccharide (LPS) component present in the outer membranes of Gram-negative bacteria.

  7. Leave the Body/Return to Reservoir:    - Pathogens spread to other parts of the body via various systems (e.g., blood, lymphatic).    - Exit pathways include direct contact, indirect contact, or via vectors.

The Role of the Immune System

  • The immune system serves as a defense mechanism protecting the host from pathogens.

  • Its main aims include:   - Identifying and eliminating pathogens that invade the body.

Summary

  • Pathogens present significant threats to human health, leading to potential infections and diseases.

  • The immune system's role is critical in both preventing and responding to pathogen invasions. It attempts to eliminate those causing infections.