Our immune system is essential for our survival. Without an immune system, our bodies would be open to attack from bacteria, viruses, parasites, and more. It is our immune system that keeps us healthy as we drift through a sea of pathogens.
This vast network of cells and tissues is constantly on the lookout for invaders, and once an enemy is spotted, a complex attack is mounted. The immune system is spread throughout the body and involves many types of cells, organs, proteins, and tissues. Crucially, it can distinguish our tissue from foreign tissue — self from non-self. Dead and faulty cells are also recognized and cleared away by the immune system. If the immune system encounters a pathogen, for instance, a bacterium, virus, or parasite, it mounts a so-called immune response.
How Our Bodies Get Immunity
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, by this time, we have been exposed to more pathogens and developed more immunity.
That is why teens and adults tend to get sick less often than children.
Once an antibody has been produced, a copy remains in the body so that if the same antigen appears again, it can be dealt with more quickly. This is what makes some diseases, such as chickenpox, acquired only once as the body has a chickenpox antibody stored.
There are several ways the body develops immunity against various disease causing pathogens. They include:
We are all born with some level of immunity to invaders. Human immune systems, similarly to those of many animals, will attack foreign invaders from day one. This innate immunity includes the external barriers of our body — the first line of defense against pathogens — such as the skin and mucous membranes of the throat and gut. This response is more general and non-specific. If the pathogen manages to dodge the innate immune system, adaptive or acquired immunity kicks in.
Adaptive (acquired) immunity
This protection against pathogens develops as we go through life. As we are exposed to diseases or get vaccinated, we build up a library of antibodies to different pathogens. This is sometimes referred to as immunological memory because our immune system remembers previous enemies.
Immunization introduces antigens or weakened pathogens to a person in such a way that the individual does not become sick but still produces antibodies. Because the body saves copies of the antibodies, it is protected if the threat should reappear later in life.
Some Important Cells That Make up the Immune System
White blood cells are also called leukocytes. They circulate in the body in blood vessels and the lymphatic vessels that parallel the veins and arteries.
White blood cells are on constant patrol and looking for pathogens. When they find a target, they begin to multiply and send signals out to other cell types to do the same. Our white blood cells are stored in different places in the body, which are referred to as lymphoid organs which include the thymus, spleen, bone marrow and lymph nodes.
There are two main types of white blood cells;
1. Phagocytes – These cells surround and absorb pathogens and break them down, effectively eating them. There are several types, including:
- Neutrophils — these are the most common type of phagocyte and tend to attack bacteria.
- Monocytes — these are the largest type and have several roles.
- Macrophages — these patrol for pathogens and also remove dead and dying cells.
- Mast cells — they have many jobs, including helping to heal wounds and defend against pathogens.
2. Lymphocytes – Lymphocytes help the body to remember previous invaders and recognize them if they come back to attack again. Lymphocytes begin their life in bone marrow. Some stay in the marrow and develop into B lymphocytes (B cells), others head to the thymus and become T lymphocytes (T cells). These two cell types have different roles.
B lymphocytes produce antibodies and help alert the T lymphocytes. T lymphocytes destroy compromised cells in the body and help alert other leukocytes.
How the Immune system works
The immune system needs to be able to tell self from non-self. It does this by detecting proteins that are found on the surface of all cells. It learns to ignore its own or self proteins at an early stage.
An antigen is any substance that can spark 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 faulty or dead. Initially, a range of cell types works together to recognize the antigen as an. invader.
Once B lymphocytes spot the antigen, they begin to secrete antibodies (antigen is short for “antibody generators”). Antibodies are special proteins that lock on to specific antigens. Each B cell makes one specific antibody. For instance, one might make an antibody against the bacteria that cause pneumonia, and another might recognize the common cold virus.
The B lymphocytes then alert the T lymphocytes to take up action against the spotted antigen.There are distinct 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-eating phagocytes.
Killer T cells (cytotoxic T lymphocytes) as the name suggests, attack and kill 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 destroy the infection.