Infection and the immune system

Table of contents

Introduction of the infections

An infection occurs when a pathogen, such as a virus, bacterium, parasite, or fungus, enters the body and starts to multiply, causing damage to the body’s tissues.
Infections can be acute (short-term) or chronic (long-lasting) and can occur anywhere in the body.
The body has multiple defence mechanisms, but sometimes pathogens can evade these defences, leading to illness.
Infections are a major cause of morbidity (disease) and mortality (death) worldwide, but effective hygiene practices, vaccination, and timely medical interventions can significantly reduce their impact.

Classification of Infections

Based on the Type of Pathogen:
- Bacterial Infections:
  - Bacteria are single-celled organisms that can multiply rapidly and produce toxins, leading to tissue damage. They can cause various illnesses, including pneumonia, tuberculosis, urinary tract infections (UTIs), and food poisoning.
  - Examples: Streptococcus pneumoniae (pneumonia), Escherichia coli (food poisoning), Mycobacterium tuberculosis (tuberculosis).
- Viral Infections:
  - Viruses are much smaller than bacteria and cannot reproduce on their own. They invade a host cell, hijack its machinery, and reproduce within the host. This leads to the destruction of the infected cells and the spread of new virus particles.
  - Examples: Influenza virus (flu), Human Immunodeficiency Virus (HIV), Hepatitis B and C, SARS-CoV-2 (COVID-19).
- Fungal Infections:
  - Fungi are organisms that include yeasts, molds, and mushrooms. They can infect the skin, lungs, or other organs, especially in people with weakened immune systems.
  - Examples: Candida albicans (thrush), Aspergillus (aspergillosis), Dermatophytes (ringworm).
- Parasitic Infections:
  - Parasites live on or inside a host and derive nutrients at the host’s expense. Some parasites live inside the body, while others may infect the skin or other tissues.
  - Examples: Plasmodium falciparum (malaria), Giardia lamblia (giardiasis), Taenia solium (tapeworm).
- Prion Infections:
  - Prions are infectious proteins that cause abnormal folding of other proteins in the brain, leading to diseases that are typically neurodegenerative and incurable.
  - Examples: Prion diseases such as Creutzfeldt-Jakob disease and mad cow disease (bovine spongiform encephalopathy).
Based on the Duration of Infection:
- Acute Infection:
  - An infection that develops suddenly, typically with rapid onset and severe symptoms, but resolves in a short period, often with treatment.
  - Examples: Influenza, a common cold, strep throat, pneumonia.
- Chronic Infection:
  - Chronic infections persist for a long time (months to years) and can cause ongoing or intermittent symptoms. These infections may not be fully eradicated and can sometimes worsen over time.
  - Examples: Hepatitis B or C, HIV, tuberculosis.
- Latent Infection:
  - These infections remain dormant in the body after the initial infection has been cleared. The pathogen can reactivate later, often triggered by factors like immune suppression.
  - Examples: Herpes simplex virus (cold sores, genital herpes), varicella-zoster virus (chickenpox, shingles).
Based on the Area of the Body Affected:
- Local Infections:
  - Affects a specific area or part of the body. Symptoms are typically confined to that site, and the infection is often easier to treat.
  - Examples: Skin infections (e.g., impetigo), ear infections (e.g., otitis media), urinary tract infections.
- Systemic Infections:
  - Spread throughout the body, often through the bloodstream, leading to widespread symptoms. These infections are usually more serious and require more extensive treatment.
  - Examples: Sepsis, malaria, HIV, influenza.
- Opportunistic Infections:
  - Infections that take advantage of a weakened immune system to cause disease. These pathogens may not be harmful in healthy individuals, but they can lead to severe illness in immunocompromised people.
  - Examples: Pneumocystis jirovecii (pneumonia in HIV patients), Candida albicans (thrush in immunocompromised individuals).

Sources of Infection

Infections are caused by exposure to pathogens, which can come from several sources. Understanding these sources is crucial for prevention and control.

Human-to-Human Transmission:
- Direct Contact: This includes physical interactions such as handshakes, kissing, or sexual contact, where pathogens are transferred from one person to another.
  - Examples: HIV, Hepatitis B, Herpes simplex virus (cold sores), Staphylococcus aureus (skin infections).
- Indirect Contact: Pathogens can be transferred from surfaces contaminated by infected individuals, such as doorknobs, towels, and shared objects.
  - Examples: Norovirus (gastroenteritis), Influenza.
Animal-to-Human Transmission (Zoonotic Infections):
- Bites or Scratches: Some animals, especially those with infectious diseases, can transmit pathogens through bites or scratches.
  - Examples: Rabies (from bites), Toxoplasmosis (from cats), Cat scratch fever.
- Direct Contact: Handling infected animals or their products (such as raw meat) can expose humans to pathogens.
  - Examples: Hantavirus (from rodents), Salmonella (from poultry).
- Insect-borne Diseases: Insects such as mosquitoes, ticks, and fleas can carry and transmit infections from animals to humans.
  - Examples: Malaria (mosquitoes), Lyme disease (ticks), Plague (fleas).
Environmental Sources:
- Water and Food: Contaminated water and food can transmit various infections, particularly in unsanitary conditions.
  - Examples: Cholera, Salmonella (food poisoning), Hepatitis A (contaminated water).
- Soil: Soil can harbor certain pathogens that can infect individuals through direct contact, particularly in agricultural or outdoor environments.
  - Examples: Tetanus (from contaminated soil), Hookworm (from soil).
Airborne Transmission:
- Pathogens can travel through the air in droplets or aerosols, especially when an infected person coughs, sneezes, or talks.
  - Examples: Tuberculosis, Measles, COVID-19.
Vector-borne Transmission:
- Pathogens are transmitted through the bites of infected vectors, such as mosquitoes, ticks, or fleas.
  - Examples: Malaria (mosquitoes), Dengue fever (mosquitoes), Lyme disease (ticks).

Method of Transmission

Contact Transmission:
- Direct Contact: Physical interaction with an infected person or bodily fluids (e.g., sexual contact, skin-to-skin).
- Indirect Contact: Transmission occurs via contaminated objects or surfaces (e.g., shared towels, doorknobs, phone screens).
Droplet Transmission:
- Pathogens are spread through droplets that are expelled when a person coughs, sneezes, talks, or breathes. These droplets can travel short distances before settling on nearby surfaces or being inhaled by others.
- Examples: Flu, Common cold, COVID-19.
Airborne Transmission:
- Pathogens become suspended in the air as tiny particles or aerosols, which others can inhale. This transmission method allows pathogens to travel long distances.
- Examples: Tuberculosis, Measles.
Fecal-Oral Transmission:
- Pathogens are transmitted when faecal matter from an infected person contaminates food, water, or surfaces and is ingested by another person.
- Examples: Cholera, Hepatitis A, Giardiasis.
Vertical Transmission:
- Pathogens are passed from mother to child, either during pregnancy, childbirth, or breastfeeding.
- Examples: HIV, Syphilis, Herpes simplex virus.

Precautions to Prevent Infections

Personal Hygiene:
- Hand Hygiene: Wash hands thoroughly with soap and water for at least 20 seconds. Use alcohol-based hand sanitizer when soap and water are unavailable.
- Respiratory Hygiene: Cover your mouth and nose with a tissue or elbow when coughing or sneezing.
- Avoid Touching Face: Refrain from touching the face, particularly the eyes, nose, and mouth, to reduce the risk of transferring pathogens.
Vaccination:
- Vaccines prevent certain infections and provide immunity to diseases like measles, chickenpox, flu, and hepatitis.
- Importance: Vaccination protects individuals and contributes to herd immunity, reducing the spread of diseases within the community.
Food and Water Safety:
- Food Handling: Wash hands and surfaces before preparing, cook food to the appropriate temperature, and safely store food.
- Water Safety: Drink water from clean, safe sources. Avoid consuming unclean water, particularly in developing areas.
Use of Protective Equipment:
- Masks: Wear masks in crowded or high-risk environments to prevent airborne transmission of diseases like COVID-19.
- Gloves: Use gloves when handling potentially contaminated materials or in healthcare settings.
- Insect Repellents: Apply insect repellent to prevent mosquito and tick bites.
Sexual Health Practices:
- Use condoms to prevent sexually transmitted infections (STIs) and practice safe sexual health habits.
Avoid Close Contact with Sick People:
- Stay away from individuals showing symptoms of contagious diseases, especially those with fever, cough, or gastrointestinal symptoms.
Regular Cleaning and Disinfection:
- Clean and disinfect frequently touched surfaces regularly to minimize the spread of pathogens in homes, offices, and healthcare settings.
Strengthening Immune System:
- Eat a balanced diet, exercise regularly, sleep well, and reduce stress to support overall immune function.

Introduction of the Immune System

The immune system is an intricate network of cells, tissues, and organs that protect the body from harmful invaders like bacteria, viruses, fungi, parasites, and even cancer cells.
This system is essential for preventing infections and diseases, maintaining tissue health, and ensuring the body functions optimally.
The immune system can recognise and eliminate pathogens while distinguishing between self (healthy cells) and non-self (foreign invaders).

There are two main categories of the immune system:

Innate Immunity – The first line of defence is non-specific and always ready to respond immediately.
Adaptive Immunity – A more specialized and targeted immune response that develops after exposure to specific pathogens.

Antigens

Definition: An antigen is any substance the immune system recognizes as foreign or dangerous. Antigens can be proteins, lipids, carbohydrates, nucleic acids found on the surface of pathogens (like bacteria and viruses) or even substances like pollen, toxins, and transplanted organs.
Types of Antigens:
- Exogenous Antigens: Originate from outside the body. Examples include pathogens like viruses, bacteria, and allergens.
- Endogenous Antigens: These are produced by the body’s own cells but are abnormal or altered (for example, cancer cells or infected cells).
- Autoantigens: These are self-antigens that mistakenly trigger an immune response in autoimmune diseases (like in lupus or rheumatoid arthritis).
How Antigens Trigger Immune Response: When an antigen enters the body, specialized immune cells, particularly dendritic cells, capture and process the antigen. They present it to other immune cells like T-cells, which recognize the specific antigen, activating the immune system to mount a defense.

Antibodies

Definition: Antibodies, or immunoglobulins (Ig), are Y-shaped proteins produced by B-cells of the immune system. Antibodies are highly specific to the antigen that triggered their production and serve as a powerful tool in fighting infections.
Structure: Antibodies have two regions:
- Fab Region (Antigen-Binding): This part binds to the antigen. Each antibody has two identical antigen-binding sites that recognize and bind to a particular antigen.
- Fc Region (Constant Region): This region determines the class of the antibody and interacts with other immune cells like macrophages, neutrophils, and natural killer cells.
Types of Antibodies:
- IgG: The most abundant antibody in blood and tissue fluids, critical for long-term immunity.
- IgA: Found in mucosal areas (like the respiratory and gastrointestinal tracts) and in secretions like saliva and breast milk. It helps protect mucosal surfaces.
- IgM: The first antibody produced during an immune response. It is especially effective in neutralizing pathogens.
- IgE: Involved in allergic reactions and responses to parasitic infections.
- IgD: Functions mainly as a receptor on B-cells that triggers their activation.
Role of Antibodies: Antibodies neutralize antigens by:
- Neutralization: Preventing the pathogen from entering host cells.
- Opsonization: Marking the pathogen for destruction by other immune cells.
- Agglutination: Clumping pathogens together, making it easier for immune cells to remove them.

Types of Immunity

1. Innate Immunity (Natural Immunity):

Overview: Innate immunity is the body’s initial, rapid, and non-specific defense against pathogens. It is the first line of defense and operates immediately or within hours of exposure to an antigen.
Key Components:
- Physical Barriers: Skin and mucosal membranes act as physical barriers, preventing pathogen entry.
- Phagocytic Cells: Cells like macrophages, neutrophils, and dendritic cells recognize and engulf pathogens.
- Inflammatory Response: Inflammation is triggered by immune cells to isolate infected areas, causing redness, heat, swelling, and pain.
- Natural Killer (NK) Cells: These cells identify and destroy infected or cancerous cells.
- Complement System: A group of proteins in the blood that work together to lyse pathogens and enhance the immune response.

2. Adaptive Immunity (Acquired Immunity):

Overview: Adaptive immunity is highly specific and adaptable. It takes time to develop (days to weeks) and provides long-lasting protection by recognizing and “remembering” specific pathogens.
Key Components:
- B-cells: These produce antibodies in response to antigen exposure. They also develop into memory B-cells, which can quickly respond if the same pathogen is encountered again.
- T-cells: These are involved in directly killing infected cells or helping activate other immune cells.
  - Helper T-cells (CD4+): Activating B-cells and other immune cells.
  - Cytotoxic T-cells (CD8+): Directly kill infected or abnormal cells.
- Memory Cells: Both memory B-cells and memory T-cells are formed after an initial infection. If the same pathogen is encountered in the future, these cells mount a faster and more efficient immune response.

Role of the Immune System

The immune system’s role is to protect the body from diseases caused by pathogens, but its functions are far-reaching and crucial for overall health:

Recognition of Foreign Invaders:
- The immune system uses specialized receptors to recognize molecular patterns present on pathogens, called pathogen-associated molecular patterns (PAMPs), which are different from normal body cells. This helps the system identify harmful invaders.
Phagocytosis:
- Phagocytes, such as macrophages and neutrophils, engulf and digest pathogens and debris. This process is known as phagocytosis.
Activation of the Adaptive Immune System:
- After the innate immune system has identified an infection, it activates the adaptive immune system by presenting antigens by dendritic cells to T-cells.
- Helper T-cells coordinate the response by activating B-cells to produce antibodies and cytotoxic T-cells to destroy infected cells.
Destruction of Pathogens:
- Once a pathogen is recognized, it can be destroyed in several ways:
  - Antibodies neutralize or mark pathogens for destruction.
  - Cytotoxic T-cells directly kill infected cells.
  - Macrophages and neutrophils engulf and digest the pathogen.
Memory and Protection:
- After an infection is cleared, memory cells (both B and T cells) remain in the body. This allows for faster recognition and a more robust immune response if the pathogen is encountered again in the future, providing immunological memory.
Immune Surveillance:
- The immune system continually surveys the body for signs of abnormal cells, such as cancerous or mutated cells. Natural killer cells and cytotoxic T-cells play a role in eliminating these abnormal cells.