Diseases and immunity

Disease is a condition where the body is unable to function normally because its internal environment is not stable, a state known as a failure of homeostasis. Diseases can be broadly classified into two types. Non-infectious diseases are not caused by living organisms and cannot be passed from person to person; examples include genetic disorders like cystic fibrosis or lifestyle diseases like coronary heart disease. Infectious diseases are caused by pathogens and can be transmitted between individuals. A pathogen is a microorganism that causes disease. There are several ways pathogens can spread.

Before pathogens can cause disease, they must first enter the body. The body has a range of natural barriers that form the first line of defence, preventing entry. These defences are non-specific, meaning they act against all types of pathogens. They can be categorised as mechanical or chemical barriers. The skin is a key mechanical barrier, forming a tough, continuous outer layer that pathogens cannot easily penetrate. Hairs in the nose help to trap larger particles. Chemical barriers include the hydrochloric acid in the stomach, which has a very low pH (around 2) that kills most pathogens swallowed in food or drink. Mucus, a sticky substance produced in the respiratory tract, traps pathogens, which are then wafted away by cilia. Tears and saliva contain an enzyme called lysozyme, which breaks down the cell walls of bacteria.

If pathogens manage to breach the first line of defence and enter the body, the second line of defence is activated. This involves a type of white blood cell called a phagocyte. Phagocytes carry out a process called phagocytosis, which is a non-specific response to infection. This means they will attack any cell or particle they identify as foreign. The process begins when a phagocyte detects chemicals released by a pathogen and moves towards it. The phagocyte's cell membrane then flows around the pathogen, engulfing it and enclosing it within a vesicle called a phagosome. Inside the cytoplasm, lysosomes (vesicles containing digestive enzymes) fuse with the phagosome. The powerful enzymes are released and break down, or digest, the pathogen into harmless substances, which are then absorbed by the cell.

If an infection is not cleared by phagocytes, the third line of defence, the specific immune response, is activated. This is carried out by another type of white blood cell called a lymphocyte. There are two main types, B-lymphocytes and T-lymphocytes. This response is 'specific' because it targets one particular type of pathogen. Every pathogen has unique molecules on its surface called antigens. When a pathogen enters the body, a B-lymphocyte with a receptor that matches the pathogen's antigen is selected and activated. This B-lymphocyte then divides rapidly by mitosis (a process called clonal selection) to produce two types of cells: plasma cells and memory cells. Plasma cells are antibody factories, producing huge numbers of specific proteins called antibodies. These antibodies circulate in the blood and bind to the pathogen's antigens. This does not kill the pathogen directly, but it can neutralise it (e.g., by blocking it from entering host cells) or, more commonly, cause the pathogens to clump together, making them easier for phagocytes to find and destroy.

The specific immune response not only clears an infection but also provides long-term protection, a state known as immunity. This is due to immunological memory. During the first-time infection, called the primary response, it takes time for the correct B-lymphocytes to be activated and produce enough antibodies to fight the pathogen. During this lag phase, the person experiences symptoms of the disease. As well as plasma cells, the activated B-lymphocytes also produce long-lived memory cells. These memory cells remain in the blood for months or even years. If the same pathogen enters the body again, the memory cells recognise the antigen immediately. They trigger a secondary response, which is much faster and stronger than the primary one. Memory cells rapidly divide to produce a large number of plasma cells, leading to a massive and rapid production of antibodies. This response is so quick that the pathogen is destroyed before it can multiply and cause symptoms. The person is therefore immune to the disease.

Immunity can be acquired in different ways, which can be classified as active or passive, and natural or artificial.

Active Immunity: The body is challenged with an antigen and produces its own antibodies and memory cells. This type of immunity is long-lasting.

• Natural Active Immunity is acquired when you catch a disease, recover, and your body makes memory cells against it.
• Artificial Active Immunity is acquired through vaccination, where a harmless form of an antigen is injected to stimulate your body to make memory cells.

Passive Immunity: The body is given ready-made antibodies from another source. It does not produce its own antibodies or memory cells. This type of immunity is immediate but temporary, as the antibodies are eventually broken down.

• Natural Passive Immunity is acquired by a baby from its mother. Antibodies pass across the placenta before birth and are present in breast milk after birth.
• Artificial Passive Immunity is acquired from an injection of antibodies (antitoxin) taken from another person or animal, providing emergency protection against a disease like tetanus.

Vaccination is a safe and effective way of producing artificial active immunity and is one of the most important tools in modern medicine for preventing infectious diseases. A vaccine contains a safe version of a pathogen, which can be a dead pathogen, a weakened (attenuated) live pathogen, or simply the antigens from the pathogen's surface. When this is introduced into the body (usually by injection), the antigens trigger a primary immune response. The immune system produces antibodies and, crucially, memory cells, but without the person suffering from the actual disease. If the vaccinated person is later exposed to the live pathogen, their body can mount a rapid and powerful secondary immune response. The memory cells ensure that specific antibodies are produced so quickly and in such large quantities that the pathogen is destroyed before it can cause illness. When a large percentage of a population is vaccinated, it creates 'herd immunity', which helps to protect individuals who cannot be vaccinated (e.g., babies or people with weak immune systems) because it reduces the pathogen's ability to spread.