Immune System

Pathogens, such as viruses, bacteria, and toxins, cause these infections. These pathogens are spread from person to person via coughing and sneezing or through contaminated food or water supplies.

The function of the immune system

The main parts of the immune system are the white blood cells, antibodies, bone marrow, the complement and lymphatic systems, and two organs, the spleen, and thymus.

The role of bone marrow in the immune system

Bone marrow is spongy tissue located inside of bones. The bone marrow helps produce red and white blood cells and platelets. Platelets are cell fragments found in the blood that is used to stop bleeding via clotting. Red blood cells play a recently discovered role in the immune system in which they are also contacted by cytokines, a type of protein, in hopes of triggering inflammation.

The lymphatic system aids the immune system

The lymphatic system helps the body both with its immune system and the circulatory system. The lymphatic system consists of organs, vessels, and tissues that push a clear fluid, known as lymph, throughout the body. Lymph comes from draining various cells and tissues, including damaged cells, cancer cells, viruses, and bacteria. This lymph fluid helps transport the lymphocytes, white blood cells, to where they need to go in the body to destroy invaders. Alongside lymph, the body contains around 600 bean-shaped glands known as lymph glands. These lymph glands help produce and store the lymphocytes and other immune cells. Moreover, they filter out the lymph's damaged and cancerous cells.

Fig. 1 - The lymphatic system in the human body

The role of the spleen in the immune system

The spleen is an organ in the shape of a bean. It has red and white tissue, each colour having a different function. The red tissue stores blood and removes damaged blood cells, whilst the white tissue is part of the lymphatic system and helps produce white blood cells, producing antibodies.

The role of the thymus in the immune system

The thymus is a small pyramid-shaped organ located in the chest in front of the heart. This organ creates T cells by transforming white blood cells the bone marrow produces into them. As organisms age, the thymus begins to shrink and turn into fatty tissue, and the rate of T cell production declines. Organisms with a damaged or missing thymus are vulnerable to infections.

What is an overactive immune system?

Occasionally, the immune system can react heavily to things in the environment that are considered harmless such as dust, mould, pollen, pet dander, or foods. These things that cause the immune system to react are called allergens, and when the body responds to these allergens, it is called an allergic reaction.

Function of the immune system

There are two types of the immune system, the innate immune system and the adaptive immune system. Each of these immune systems works differently to help keep the body safe.

The innate immune system

The innate immune system, or the non-specific immune system, works to prevent all types of infections from happening. This immune system uses our skin and the body's openings, like eyes and nose, to prevent disease. Skin, tears, the nasal cavity, and saliva contain lysozymes that work by bursting the cell walls of the different viruses. Another way that the innate immune system helps kill various pathogens is by sending consumed pathogens from food into the stomach, where the highly acidic stomach acid destroys them. If a virus does make it into the body, then other processes take place to fight it off. One of the processes is inflammation, where white blood cells consume the pathogens, resulting in the swelling of the body part. Another process is fever, where the higher temperatures can either help kill the pathogens or slow their reproduction.

The complement system is part of the innate immune system and helps activate plasma proteins found in the blood, with either pathogens or a pathogen-bound antibody. Since it is part of the innate immune system, it helps activate inflammation, label pathogens and cells that must be destroyed, and kill those pathogens and cells.

The adaptive immune system

The adaptive immune system, also known as the specific immune system, is the body's particular targeting of pathogens. Our bodies know which cells belong to us, so cells that do not belong to the body are spotted and attacked by the other cells. If a pathogen is attacked for the first time, the body is typically able to remember it and can fight it off easier next time. Antigens, cells that live on the surface of pathogens, usually trigger the adaptive immune system to respond. When the body creates antibodies, a type of protein, they attach to the antigens and destroy them.

Fig. 2 - Antibody and antigens bond

Different types of immunity

There are two types of immunity, humoral and cell-mediated immunity, each playing its role in the immune system.

What is humoral immunity?

Humoral immunity, also known as antibody-mediated immunity, is activated when antigens are found in the body. Once these antigens are detected, B cell lymphocytes, a type of white blood cell, perform a differentiation that creates memory B cells and effector B cells. Memory B cells remember the current pathogen, making the production of antibodies faster in the future, and the effector B cells release antibodies.

What is cell-mediated immunity?

Cell-mediated immunity relies on T cells, macrophages, and cytokines to help destroy the antigens. T cells, a type of white blood cell, are created by the thymus and stem cells in the bone marrow and target specific antigens. There are two types of T cells, cytotoxic T cells and helper T cells. The cytotoxic T cells work by releasing toxins into infected cells, and helper T cells work by notifying other cells about the antigens in the body. Macrophages are specialized cells that help remove diseased and damaged cells from the body. They also help T cells by presenting them antigens and releasing cytokines. Cytokines are proteins that help cells communicate and start the inflammatory response.

How can an immune system be damaged?

The immune system can be damaged either by viruses or by lifestyle choices. Lifestyle choices, consisting of smoking, drinking, or poor nutrition, impact the immune system because they can harm the production and activity of immune cells. Viruses, like HIV, damage the immune system by attacking and destroying T cells, making it harder for people with those viruses to fight off other infections such as a common cold.