How the immune system works

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 โ€” 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. Later, we will explain how this works, but first, we will introduce some of the main characters in the immune system.

White blood cells
A white blood cell (yellow), attacking anthrax bacteria (orange). The white line at the bottom is 5 micrometers long.
Image credit: Volker Brinkmann
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. These include the following:

Thymus โ€” a gland between the lungs and just below the neck.
Spleen โ€” an organ that filters the blood. It sits in the upper left of the abdomen.
Bone marrow โ€” found in the center of the bones, it also produces red blood cells.
Lymph nodes โ€”small glands position throughout the body, linked by lympha vessels.
There are two main types of leukocyte:
1. Phagocytes
These cells surround and absorb pathogens and break them down, effectively eating them. There are several types, including:
Neutrophils โ€” these are the most common type of phagocyte and tend t attack bacteria.
Monocytes โ€” these are the largest typ and have several roles.
Macrophages โ€” these patrol for pathogens and also remove dead and dying cells.
Mast cells โ€” 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 โ€” they produce antibodies and help alert the T lymphocytes.
T lymphocytes โ€” they destroy compromised cells in the body and he alert other leukocytes.
How an immune response works
B lymphocytes secrete antibodies (pictured) that lock onto antigens.
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.
The role of B lymphocytes
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.
Antibodies are part of a large family of chemicals called immunoglobulins, which play many roles in the immune response:
Immunoglobulin G (IgG) โ€” marks microbes so other cells can recognize and deal with them.
IgM โ€” is expert at killing bacteria.
IgA โ€” congregates in fluids, such as tears and saliva, where it protects gateways into the body.
IgE โ€” protects against parasites and i also to blame for allergies.
IgD โ€” stays bound to B lymphocytes, helping them to start the immune response.
Antibodies lock onto the antigen, but they do not kill it, only mark it for death. The killing is the job of other cells, such as phagocytes.
The role of T lymphocytes
There are distinct types of T lymphocytes:
Helper T cells (Th cells) โ€” they 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) โ€” 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 destroy the infected cells.