Health, disease and the development of medicines revision notes

Understanding Immunisation

Immunisation (vaccination) is a method of acquiring immunity to a disease. It involves introducing a vaccine into the body, which contains inactive or weakened forms of a pathogen, or just its antigens. The key is that the vaccine cannot cause the disease but still presents the unique molecular markers (antigens) of the pathogen to the immune system.

Immune System Response to Vaccination

When a vaccine is administered, the immune system detects the foreign antigens. This triggers white blood cells, specifically lymphocytes, to produce specific antibodies. These antibodies are designed to bind to and neutralise the specific antigens. Crucially, the body also produces memory cells. These memory cells remain in the bloodstream, enabling a rapid and strong immune response if the actual pathogen is encountered later, preventing illness.

Benefits of Immunisation

The primary benefit of immunisation is the protection against serious diseases. By creating immunity without exposure to the full-blown disease, it prevents individuals from suffering the symptoms and potential complications of infections. Widespread immunisation also contributes to 'herd immunity', protecting vulnerable individuals who cannot be vaccinated.

Benefits of Immunisation

Immunisation offers significant advantages for both individuals and communities. For individuals, it provides protection against specific diseases, preventing severe illness, complications, hospitalisation, and death. It trains the immune system to recognise and fight off pathogens quickly. From a public health perspective, widespread immunisation contributes to herd immunity, reducing disease transmission and protecting vulnerable groups who cannot be vaccinated.

Limitations of Immunisation

While highly effective, immunisation also has limitations. Some individuals may experience mild side effects like soreness at the injection site, fever, or fatigue. More severe reactions are rare but possible. Vaccines are not 100% effective for everyone, meaning a small percentage of vaccinated individuals can still contract the disease. Pathogens can mutate, requiring new vaccine development (e.g., influenza), and logistical challenges exist in vaccine distribution and administration in some areas. Public misinformation can also impact uptake.

Herd Immunity Explained

Herd immunity, or community immunity, describes a form of indirect protection from infectious disease that occurs when a large percentage of a population has become immune to an infection, thereby providing a measure of protection for individuals who are not immune. For example, if 95% of a population is vaccinated against measles, the remaining 5% (including infants and medically vulnerable individuals) are less likely to be exposed to the virus because its spread is significantly curtailed within the community.

Physical Barriers Against Pathogens

Physical barriers are the body's first line of defence against pathogens. Key examples include:

• Skin: Forms a tough, outer, waterproof layer that pathogens cannot easily penetrate. It also continuously sheds outer cells, removing any attached pathogens.
• Mucus: A sticky substance produced by mucous membranes lining the respiratory, digestive, and urogenital tracts. It traps pathogens effectively.
• Cilia: Tiny hair-like structures found on cells lining the respiratory tract. They rhythmically beat, moving the pathogen-laden mucus upwards towards the throat, where it can be swallowed and destroyed by stomach acid, or expelled.

Chemical Defences Against Pathogens

Chemical defences involve substances that destroy or neutralise pathogens. These include:

• Lysozymes: Enzymes found in tears, saliva, and sweat. They break down the cell walls of certain bacteria, leading to their destruction.
• Hydrochloric acid: Present in the stomach, it creates a highly acidic environment (low pH) which is lethal to most bacteria and other microorganisms ingested with food and drink.

Weakened Immune System

An existing disease often diminishes the immune system's effectiveness. This makes the body less capable of recognising and fighting off new pathogens, increasing the risk of subsequent infections. Conditions like HIV/AIDS directly target immune cells, leaving the body defenceless against common illnesses.

Physical Damage and Entry Points

Some diseases cause physical damage to tissues and organs. This damage can create openings or weakened areas that pathogens can exploit to enter the body or establish new infections. For example, a severe flu infection can damage the respiratory lining, making an individual more prone to bacterial pneumonia.

Chronic Stress and Nutritional Deficiencies

Long-term illnesses can lead to chronic stress on the body and may result in nutritional deficiencies due to altered metabolism or reduced appetite. These factors further compromise the immune system and overall health, reducing resistance to new diseases. Chronic inflammation associated with some diseases can also suppress immune responses.

Overwhelmed Body Systems

When the body is already battling one disease, its resources and energy are diverted to fight that existing threat. This leaves fewer resources available to mount an effective response against a new pathogen, making it easier for a secondary infection to take hold and cause illness.