Without an immune system, our bodies would be exposed to attack by bacteria, viruses, parasites, and more. It is our immune system that keeps us healthy as we move through a sea of pathogens. This vast network of cells and tissues is constantly on the lookout for invaders, and once an enemy is detected, a complex attack is mounted.
Immune system – Characteristics
The immune system is spread throughout the body and involves many types of cells, organs, proteins, and tissues. Fundamentally, it can distinguish our tissue from foreign tissue: oneself and another not. Dead and defective cells are also recognized and eliminated by the immune system.
If the immune system encounters a pathogen, for example a bacterium, virus or parasite, a so-called immune response occurs. White blood cells are also called leukocytes. They circulate in the body in blood vessels and lymphatic vessels that resemble veins and arteries.
White blood cells look for pathogens. They are stored in different places in the body, which are known as lymphoid organs. These include the following:
- Thymus: a gland between the lungs and just below the neck.
- Spleen: an organ that filters blood. It sits in the upper left part of the abdomen.
- Bone marrow: It is found in the center of the bones and also produces red blood cells.
- Lymph nodes: small glands located throughout the body, linked by lymphatic vessels.
Immune system – Leukocytes
There are two main types of leukocytes:
These cells surround and absorb pathogens and break them down, effectively feeding them. There are several types, including:
- Neutrophils – These are the most common types of phagocytes and tend to attack bacteria.
- Monocytes – These are the largest types and have several roles.
- Macrophages – patrol for pathogens and also kill dead and dying cells.
- Masts – They have many jobs, including helping heal wounds and fending off pathogens.
Lymphocytes help the body remember previous invaders and recognize them if they attack again.
Lymphocytes begin their life in the bone marrow. Some remain in the marrow and become B lymphocytes (B cells), others go to the thymus and become T lymphocytes (T cells). These two types of cells have different roles:
- B lymphocytes: produce antibodies and help alert T lymphocytes.
- T lymphocytes: they are a type of white blood cell that are responsible for cell-type immunity.
Immune system – Function
B lymphocytes secrete antibodies (pictured) that stick to antigens.
The immune system needs to be able to differentiate itself. It does this by detecting proteins found on the surface of all cells. Learn to ignore your own proteins or self-proteins at an early stage.
An antigen is any substance that can elicit an immune response. In many cases, an antigen is a bacterium, fungus, virus, toxin, or foreign body. But it can also be one of our own cells that is defective or dead. Initially, a variety of cell types work together to recognize the antigen as an invader.
The role of B lymphocytes
Each B cell produces a specific antibody. For example, one could make an antibody against the bacteria that causes pneumonia, and another could recognize the common cold virus.
Antibodies are part of a large family of chemicals called immunoglobulins, which play many roles in the immune response:
- Immunoglobulin G (IgG): marks microbes so that other cells can recognize and treat them.
- IgM: is an expert in killing bacteria.
- IgA: gathers in fluids, such as tears and saliva, where it protects the entrances to the body.
- IgE: protects against parasites and is also responsible for allergies.
- IgD: remains attached to B lymphocytes, helping them to initiate the immune response.
- The antibodies lock onto the antigen, but they don’t kill it, they just mark it for death. Death is the work of other cells, such as phagocytes.
The role of T lymphocytes
There are different types of T lymphocytes:
- Helper T cells (Th cells): coordinate the immune response. Some communicate with other cells, and some stimulate B cells to produce more antibodies. Others attract more T cells or cell-feeding phagocytes.
- Killer T cells (cytotoxic T lymphocytes): As the name suggests, these T cells attack other cells. They are particularly useful for fighting viruses. They work by recognizing small parts of the virus on the outside of infected cells and destroying the infected cells.
Your skin is the first layer of defense against external pathogens. Everyone’s immune system is different but, as a general rule, it becomes stronger during adulthood as, at this time, we have been exposed to more pathogens and have developed more immunity.
That is why adolescents and adults tend to get sick less often than children. Once an antibody has been produced, a copy remains in the body, so if the same antigen reappears, it can be treated more quickly.
That’s why with some diseases, like chickenpox, you only get it once the body has a chickenpox antibody in storage, ready and waiting to destroy it the next time it arrives. This is called immunity. There are three types of immunity in humans called innate, adaptive, and passive:
We are all born with some level of immunity to invaders. Human immune systems, like those of many animals, will attack foreign invaders from day one. The first line of defense against pathogens is the skin and mucous membranes of the throat and intestine.
This answer is more general and not specific. If the pathogen manages to bypass the innate immune system, adaptive or acquired immunity is activated.
Adaptive (acquired) immunity
This protection against pathogens develops as we progress through life. As we are exposed to diseases or get vaccinated, we build a library of antibodies against different pathogens. This is sometimes known as immune memory because our immune system remembers previous enemies.
This type of immunity is “borrowed” from another source, but it does not last indefinitely. For example, a baby receives antibodies from the mother through the placenta before birth and in breast milk after birth. This passive immunity protects the baby from some infections during the first years of his life.
Immunization presents weakened antigens or pathogens to a person such that the individual does not get sick, but still produces antibodies. Because the body stores copies of the antibodies, it is protected if the threat reappears later in life.
Immune system – Disorders
Because the immune system is so complex, there are many possible ways it can go wrong. The types of immune disorder fall into three categories:
These arise when one or more parts of the immune system do not work. These can be caused in a number of ways, including age, obesity, and alcoholism. In developing countries, malnutrition is a common cause. AIDS is an example of an acquired immunodeficiency.
Dr. Samantha Robson ( CRN: 0510146-5) is a nutritionist and website content reviewer related to her area of expertise. With a postgraduate degree in Nutrition from The University of Arizona, she is a specialist in Sports Nutrition from Oxford University and is also a member of the International Society of Sports Nutrition.