Diseases and Immunity: Pathogens, Immune Responses, and Disease Prevention
Pathogens and Transmissible Diseases
Pathogen Definition: A pathogen is defined as a disease-causing organism. Pathogens include:
Viruses
Bacteria
Fungi
Parasites
Transmissible Diseases: Pathogens are passed on from one host to another; therefore, the diseases caused by pathogens are known as transmissible diseases.
Primary Modes of Pathogen Transmission:
Direct Contact: The pathogen is transferred directly from one host to another through the exchange of body fluids such as blood or semen.
Examples of diseases spread by direct contact: Human Immunodeficiency Virus (HIV), gonorrhoea, and hepatitis B and C.
Indirect Contact: The pathogen leaves the host and is carried in some way to another, uninfected individual.
Methods of Disease Transmission
Droplets in Air (Airborne Transmission):
Mechanisms: Pathogens are carried in airborne respiratory droplets expelled through coughing or sneezing.
Examples of diseases: Common cold, influenza.
Food or Water (Ingestion of Contaminated Substances):
Mechanisms: Ingestion of food or water contaminated with pathogenic organisms or fecal material.
Examples of diseases: Cholera, typhoid, dysentery.
Touching Contaminated Surfaces:
Mechanisms: Direct physical contact with surfaces contaminated by pathogens. Flies can also act as vectors by landing on contaminated material and transferring pathogens on their feet to food that is subsequently eaten.
Examples of diseases: Athlete's foot, salmonella.
Insect Bites (Vector Transmission):
Mechanisms: Invertebrate vectors (such as mosquitoes) bite hosts and inject pathogens directly into the bloodstream.
Examples of diseases: Malaria, dengue fever.
Physical and Chemical Defences Against Pathogens
Overview of Host Defences: The human body defends itself against pathogens through three primary lines of defense:
Mechanical barriers
Chemical barriers
Cells
Mechanical Barriers: Physical structures that physically obstruct pathogens from entering the body.
Skin: Covers almost all outer parts of the human body to prevent infection. If the skin is cut or grazed, it immediately begins healing itself, frequently forming a protective scab.
Hairs in the Nose: Located further up the nasal cavity to filter out particles and pathogens so they are not inhaled into the lungs.
Chemical Barriers: Chemical substances produced by specialized body cells that trap or destroy pathogens before they penetrate deeper into the body.
Mucus: Produced at various sites throughout the body (such as the respiratory tract). Pathogens become trapped in mucus and are subsequently removed via coughing, blowing the nose, or swallowing.
Stomach Acid: The stomach secretes hydrochloric acid (), which is sufficiently strong to kill pathogens swallowed in mucus or ingested through food and water.
Cellular Defences and Antibody Production
White Blood Cell Functions: Different types of white blood cells act to prevent pathogens from reaching areas of the body where they can replicate.
Phagocytosis:
Process carried out by phagocytes.
Phagocytes defend the body by engulfing and digesting pathogenic cells.
Antibody Production by Lymphocytes:
Lymphocytes produce specific proteins called antibodies.
Antibodies cause agglutination (clumping together of pathogenic cells), which restricts the movement of pathogens.
Antibodies release signaling chemicals that recruit phagocytes to destroy the clumped pathogens.
Mechanism of Antigens and Antibodies
Antigens:
All cells possess proteins and other substances projecting from their cell membranes.
These surface structures are known as antigens and are specific to each particular type of cell.
Recognition and Specificity:
Lymphocytes read the antigens on cell surfaces and recognize foreign antigens belonging to pathogens.
Each individual lymphocyte produces only one specific type of antibody.
Lymphocytes synthesize antibodies with a shape that is strictly complementary to the specific antigens present on the pathogenic cell surface.
Agglutination and Phagocyte Enhancement:
Antibodies bind directly to matching antigens on the pathogen.
Binding causes agglutination (clumping of pathogens), preventing pathogens from moving easily through tissues.
Attached antibodies release chemical signals that attract phagocytes and enhance their destructive activity.
Primary vs. Secondary Immune Response:
Primary Response: When a lymphocyte encounters a pathogen for the first time, producing specific antibodies can take several days. During this delay, the individual may experience symptoms and fall ill.
Memory Cell Formation: Lymphocytes that produce antibodies during a primary infection generate memory cells. Memory cells retain the specific biochemical instructions for manufacturing those complementary antibodies.
Secondary Response: Upon reinfection by the same pathogen species, memory cells rapidly produce specific antibodies in significantly greater quantities. The pathogens are destroyed before they can multiply and cause illness.
Pathogen Mutation Caveat:
Long-lasting immunity via memory cells does not apply to all pathogens.
Microorganisms that undergo rapid genetic mutation (such as the cold virus) alter their surface antigens.
During a secondary invasion by a mutated pathogen, existing memory cells fail to recognize the modified antigens, requiring a brand-new primary immune response.
Active Immunity vs. Passive Immunity
Active Immunity:
Definition: Immunity acquired when the body produces its own antibodies and develops memory cells for future responses to infection.
Characteristics: Slow-acting development initially, but provides long-lasting immunity.
Mechanisms of acquisition:
Natural infection with a pathogen, prompting lymphocytes to synthesize specific antibodies.
Vaccination.
Passive Immunity:
Definition: A fast-acting, short-term defence acquired when ready-made antibodies from another source or individual are introduced into the body.
Key Characteristic: The host body does not produce its own antibodies or memory cells; therefore, passive immunity is temporary.
Examples:
Maternal Transfer: Antibodies passed from mother to infant via breast milk protect infants against infections until their own immune systems mature and become responsive.
Antibody Injections: Injection of pre-formed antibodies (antitoxins/immunoglobulins) given when an immediate response is required post-exposure, such as in cases of rabies or tetanus infection.
Autoimmune Diseases and Immune System Dysfunction
Mechanism of Autoimmunity:
Normally, lymphocytes recognize the body's own cell surface antigens as self-antigens and do not initiate an immune attack.
In rare cases, immune system regulation fails, causing lymphocytes to identify self-cells as foreign.
Lymphocytes produce antibodies targeting specific healthy body cells, resulting in tissue destruction.
Case Study: Type 1 Diabetes:
In Type 1 diabetes, the immune system targets and destroys the insulin-producing cells of the pancreas.
The loss of pancreatic cells leaves the body unable to manufacture insulin or regulate blood glucose levels.
Without insulin, blood glucose concentrations rise to dangerously high levels.
Key Terminology: Antigens, Antibodies, and Antibiotics
Antigen: A chemical substance (protein or molecule) found on the outer surface of a cell.
Antibody: A complementary chemical protein produced by lymphocytes that binds to specific antigens, causing agglutination and signaling pathogenic cells for destruction.
Antibiotic: A medicinal drug that slows down or stops the growth of bacteria (ineffective against viral pathogens).
Vaccination Principles and Herd Immunity
Definition and Function of Vaccination:
Vaccination provides protection against specific diseases and enhances the immune system without requiring individuals to suffer from dangerous or fatal infections.
Population-level protection depends directly on the proportion of individuals who receive the vaccine.
Step-by-Step Mechanism of Action:
A dead, weakened, or altered form of a pathogen containing specific antigens is introduced into the body.
Because the pathogen is harmless/altered, it cannot cause clinical illness, but its surface antigens trigger an immune response.
Activation of lymphocytes occurs (which may take several days during the initial exposure).
Activated lymphocytes multiply and produce specific complementary antibodies that target and attach to the antigens.
The immune response generates long-lasting memory cells that persist in the blood for years.
If the individual encounters the live, active pathogen in the future, memory cells rapidly produce high levels of antibodies, conferring long-term immunity.
Herd Immunity:
Definition: Protection conferred upon an entire population when a sufficiently high percentage of individuals are vaccinated.
Mechanism: When most individuals are immune, there are very few susceptible hosts in which the pathogen can breed and spread. Pathogens can only multiply inside unvaccinated individuals.
Significance: Herd immunity prevents disease outbreaks, epidemics, and pandemics.
Risks of Declining Coverage: If vaccination rates fall, the proportion of susceptible hosts rises, leading to mass infections, higher transmission rates, and increased mortality.
Public Health Strategies and Disease Eradication:
Children are routinely vaccinated early in life because regular medical checkups provide opportunities to establish early immunity and sustain high population coverage.
Certain targeted vaccination campaigns aim for complete disease eradication rather than mere control.
Smallpox Eradication Example: Smallpox was officially declared eradicated worldwide in following a global vaccination initiative spearheaded by the World Health Organization (WHO) beginning in the mid-s.
Environmental and Personal Hygiene Measures
Hygienic Food Preparation:
Store food at cold temperatures to slow down the reproduction rate of bacteria and fungi.
Maintain clean surfaces by washing hands thoroughly with soap and treating food preparation areas with disinfectant products such as bleach to kill pathogens.
Cook food thoroughly at high temperatures for an adequate duration to destroy bacteria and fungi.
Keep food covered to prevent vector insects like flies from landing on it prior to consumption.
Use dedicated, separate chopping boards and utensils for raw meat to prevent cross-contamination.
Wash hands thoroughly after using the bathroom before handling food items.
Personal Hygiene:
Wash the body regularly with soap to remove physical substances that trap pathogens as well as removing the pathogens themselves from skin.
Use tissues to cover sneezes and coughs, and dispose of used tissues immediately, as pathogens remain viable on contaminated paper.
Wash hands thoroughly with soap after using the toilet.
Waste Disposal:
Dispose of food waste in sealed containers to avoid attracting flies that serve as disease vectors.
Use covered rubbish bins and arrange regular removal to landfills for controlled disposal or incineration.
Store refuse away from human living spaces prior to collection.
Sanitation:
Install functional plumbing and drainage infrastructure in homes and public facilities to safely transport human faeces and liquid waste away.
Treat raw sewage through filtration and biological treatment to eliminate solid waste and neutralize pathogens before discharging effluent into natural ecosystems.
Specific Disease Case Study: Cholera
Causative Organism: Vibrio cholerae, a pathogenic bacterium that causes acute, severe diarrhoea.
Transmission Environment:
Vibrio cholerae bacteria thrive in water contaminated with raw sewage.
Transmission occurs when humans drink contaminated water or use it to wash food.
Pathological Mechanism:
Ingested Vibrio cholerae bacteria enter the digestive tract and produce a specific protein toxin.
The cholera toxin stimulates the active secretion of chloride ions () into the lumen of the small intestine.
The accumulation of chloride ions increases the solute concentration inside the intestinal lumen, establishing an osmotic gradient.
Water moves out of the blood and tissues into the gut via osmosis.
The rapid fluid accumulation in the gut results in severe, acute diarrhoea, leading to extreme dehydration and critical loss of essential ions from the bloodstream.