Learning outcomes & key terms

About the immune system

The immune system is the body’s defence against infectious organisms and other invaders. Through a series of steps called the immune response, the immune system attacks and neutralises organisms and substances that invade body systems and cause disease

The immune system has two avenues to fight disease:

• non-specific immunity, and
• specific immunity

Non-Specific Immunity

Also known as innate immunity. This is the first line of defence once a pathogen threatens to enter the body. An example is the macrophage, which gobbles up pathogens!

• All animals have non-specific immunity
• White blood cells do the majority of the work for non-specific immunity

White Blood Cells

These are special cells because they can:

• ingest pathogens and destroy them,
• produce chemicals to destroy pathogens and they can neutralise the chemicals released by pathogens that make us sick

If a pathogen does enter the body, such as through a cut in the skin, the innate immune system will send cells to the area

The White Blood Cells will help sterilise the area, ingest and destroy any pathogens and start to heal and clot the wound to prevent any further injuries or more pathogens entering

When the innate system is at work, quite often inflammation and fever follow. Inflammation enables blood carrying immune cells to flow to the infected area more easily

White blood cells come in a variety of types: phagocytes, macrophages, lymphocytes, and neutrophils

Each type of white blood cell has a purpose to achieve a quick response in the case of pathogen infection

Specific Immunity

Specific immunity is sometimes referred to as adaptive immunity. It is slower to respond but it can learn from diseases and form a memory. Once it has seen a disease once and formed a memory, its response is rapid and effective

It is because of specific immunity that vaccinations work

• Only vertebrates have specific immunity
• Lymphocytes are involved in specific immunity

The immune system is made up of a network of cells, tissues, and organs that work together to protect the body. One of the important cells involved are white blood cells, also called leukocytes, which come in two basic types that combine to seek out and destroy disease-causing organisms or substances

There are two basic types of leukocytes:

1. phagocytes, cells that chew up invading organisms (non-specific or innate immunity) and
2. lymphocytes, cells that allow the body to remember and recognize previous invaders and help the body destroy them

A number of different cells are considered phagocytes. The most common type is the neutrophil, which primarily fights bacteria. If doctors are worried about a bacterial infection, they might order a blood test to see if a patient has an increased number of neutrophils triggered by the infection. Other types of phagocytes have their own jobs to make sure that the body responds appropriately to a specific type of invader

The two kinds of lymphocytes are B lymphocytes and T lymphocytes. Lymphocytes start out in the bone marrow and either develop there and mature into B cells, or they leave for the thymus gland, where they mature into T cells

B lymphocytes and T lymphocytes have separate functions: B lymphocytes are like the body’s military intelligence system, seeking out their targets and sending defences to lock onto them

T cells are like the soldiers, destroying the invaders that the intelligence system has identified

Leukocytes are produced or stored in many locations in the body, including the thymus, spleen, and bone marrow. For this reason, those places called lymphoid organs. There are also clumps of lymphoid tissue throughout the body, primarily as lymph nodes, that house the leukocytes

The leukocytes circulate through the body between the organs and nodes through both lymphatic vessels and blood vessels. In this way, the immune system works in a coordinated manner to monitor the body for germs or substances that might cause problems

Putting it all together: 3 lines of defence

Putting all the pieces together. Our first line of defence is barriers like skin and mucus

If pathogens get past that then the non-specific (innate and immediate) immune system is activated

If the specific immune system has seen the pathogen before it can respond more quickly and destroy the pathogen but if the germ is new to the body’s immunity, it takes a few days or a week (depending on the pathogen) to react and defend the body

Here’s how it works

When antigens (foreign substances, usually on the outside of a germ, that helps invade the body) are detected, several types of cells work together to recognize them and respond. These cells trigger the B lymphocytes to produce specific antibodies, which are specialised proteins that lock onto specific antigens

Once produced, these specific antibodies stay in a person’s body, as do mature lymphocytes with memory (called plasma cells, that can convert to rapidly make a specific antibody). If his or her immune system encounters that antigen again, the antibodies are already there to do their job, and the plasma cells switch on quickly. So if someone gets sick with a certain disease, like chickenpox, that person usually won’t get sick from it again

This is also how immunisations prevent certain diseases. An immunisation introduces the body to a particular foreign antigen in a way that doesn’t make someone sick, but does allow the body to produce antibodies (and memory cells) that will then protect the person from future attack by the germ or substance that produces that particular disease

Although antibodies can recognise an antigen and lock onto it, they are not always capable of destroying it without help. That’s the job of the T cells, which are part of the system that destroys antigens that have been tagged by antibodies or cells that have been infected or somehow changed. (Some T cells are actually called “killer cells.”) T cells also are involved in helping signal other cells (like phagocytes) to do their jobs

Antibodies also can neutralise toxins (poisonous or damaging substances) produced by different organisms. Lastly, antibodies can activate a group of proteins called complement that are also part of the immune system

Complement assists in killing bacteria, viruses, or infected cells

All of these specialised cells and parts of the immune system offer the body protection against disease. This protection is called immunity

The body has an orderly way of fighting disease through an impressive 3 lines of defense

In the next lesson we will look in further detail at how we develop immunity and the role of vaccines in preventing disease in the first place