Diseases and immunity

A disease is a condition that impairs the normal functioning of the body, moving it away from a state of 'ease'. Diseases can be infectious or non-infectious. Infectious diseases are caused by pathogens, which are microorganisms that cause disease. These diseases are transmissible, meaning they can be passed from one organism to another. Pathogens include bacteria, viruses, fungi, and protoctists. They can be transmitted in several ways: through contaminated water (e.g., Cholera), air droplets (e.g., Influenza), direct contact (e.g., Athlete's foot), vectors like insects (e.g., Malaria via mosquitoes), contaminated food (e.g., Salmonella), or body fluids (e.g., HIV).

The body has a multi-layered defence system. The first line of defence consists of non-specific barriers that prevent pathogens from entering the body. These include:

1. Mechanical barriers: The skin acts as a physical wall. Mucous membranes in the respiratory tract trap pathogens, which are then moved away by cilia.
2. Chemical barriers: Stomach acid (hydrochloric acid) kills most pathogens swallowed with food. Tears and saliva contain the enzyme lysozyme, which breaks down bacterial cell walls.

If pathogens breach these barriers, the second line of defence is activated. This involves white blood cells called phagocytes, which carry out phagocytosis. The phagocyte detects a pathogen, engulfs it into a vesicle, and releases digestive enzymes to destroy it.

If pathogens survive the first and second lines of defence, the third line, the specific immune response, is activated. This is carried out by white blood cells called lymphocytes. Every pathogen has unique molecules on its surface called antigens. When a lymphocyte with a complementary receptor protein meets a specific antigen, it is activated. There are two main types of lymphocytes involved:

1. B-lymphocytes: When activated, they divide rapidly and differentiate into plasma cells. These plasma cells produce vast quantities of proteins called antibodies.
2. T-lymphocytes: These cells have several roles, including helping to activate B-lymphocytes and destroying body cells that have been infected by viruses.

Antibodies are crucial. They have a specific shape that is complementary to one type of antigen, allowing them to bind to it. This 'locks on' to the pathogen, causing them to clump together (agglutination) which makes them easier for phagocytes to engulf, or neutralising toxins produced by the pathogen.

Immunity is the body's ability to resist a particular infection. During the first time you are infected by a pathogen (the primary response), it takes several days for the correct lymphocyte to be activated and for antibody levels to rise. During this time, you experience symptoms of the disease. As well as plasma cells, the activated B-lymphocytes also produce memory cells. These memory cells remain in the blood for a long time, sometimes for life. If the same pathogen enters the body again, the memory cells recognise the antigen immediately. They divide rapidly to produce plasma cells and more memory cells. This is the secondary response. It is much faster and stronger than the primary response, producing more antibodies so quickly that the pathogen is destroyed before you can feel any symptoms. This is long-term immunity. Vaccination makes use of this process. A vaccine contains a safe version of a pathogen – it might be dead, weakened, or just contain the antigens. This stimulates the primary immune response and the production of memory cells without causing the disease, conferring artificial active immunity.

Antibodies produced by the immune system are 'polyclonal' – they come from many different clones of B-lymphocytes. Monoclonal antibodies (mAbs) are identical antibodies, produced from a single clone of a B-lymphocyte. This means they are all specific to a single antigen. They are produced using hybridoma technology:

1. A mouse is injected with an antigen to stimulate the production of specific B-lymphocytes.
2. The B-lymphocytes are extracted from the mouse's spleen.
3. These B-lymphocytes are fused with tumour cells to create a hybridoma cell. The B-lymphocyte provides the ability to make the antibody, and the tumour cell provides the ability to divide indefinitely.
4. The hybridoma cells are cultured, and they produce large quantities of identical, monoclonal antibodies.

Uses of mAbs include pregnancy tests (detecting the hormone hCG), diagnosing diseases (by binding to antigens on pathogens), and treating diseases like cancer (by binding to cancer cell antigens and delivering drugs or triggering an immune response).