The Lymphatic System & Immune Response

Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

How does your body stay healthy when there are so many germs attacking it? Learn more about the lymphatic system that helps circulate immune cells that protect you from disease.

Lymphatic System

Your body is under constant attack from bacteria, viruses, other parasites, and sometimes, even its own cells. Most of us aren't sick all the time, though. That's thanks to the work of the lymphatic system, which makes and sends disease-fighting cells throughout the body.

White blood cells (WBCs) are a large collection of blood cells whose primary job is to get rid of the bad guys, such as bacteria, virus-infected cells, and cancer cells. There are several types of WBCs that will be covered in this lesson. All WBCs are made in the red bone marrow.. Some stay there to mature, while others go to the thymus.

Lymphatic vessels run throughout the body. They run one way, taking excess fluid out of the tissues and returning it to the blood. At various points along the lymphatic vessel highway are rest stops called lymph nodes. These are little pockets where those disease-fighting blood cells can hang out and learn about their next job. WBCs can also meet up in organs like the spleen to figure out what they need to do.

Diagram of the lymphatic system, showing lymphatic vessels, lymph nodes, and spleen.
Lymphatic system

Innate Immune Responses

A pathogen is any microscopic thing that can cause disease. When one enters your body, it first runs into WBCs that are part of the innate immune response. This response gets its name because the defensive WBCs need absolutely no training - they just eat whatever they happen to find.

The main sentries of the innate immune system are white blood cells called

  • monocytes,
  • macrophages, and
  • dendritic cells.

They go around through the blood and into tissue eating anything that looks suspicious. When you get a paper cut, you might notice some redness and swelling. This is from those WBCs migrating to the area to help kill off anyone that is trying to sneak in.

If a monocyte or dendritic cell encounters a bacterium, virus, or other parasite giving off 'bad guy' signals, it amps up the immune response. Another type of cell, an especially hungry soldier called the neutrophil, will be called over to help eat up and kill even more pathogens. Special proteins made by the macrophages, monocytes, or dendritic cells can tell normal cells in the environment to stop replicating if they detect a virus. This helps stop the virus from spreading further.

Acquired Immune Responses

Many times, the innate immune response described above is enough to stop an infection. However, while this is happening, highly-trained Special Forces, called lymphocytes are brought in. These cells are part of the acquired or adaptive immune response.

There are two main types of lymphocytes - the T cells and the B cells. T cells get their name because they mature in the thymus, while B cells mature in the bone marrow. Both types of lymphocytes have a sort of memory, meaning they can recognize pathogens after an initial infection is over.

B cells have proteins called antibodies that recognize antigens. Antigens are small portions of a pathogen that alert the body to an infection. The antibody binds to the antigen, and activates the B cell. The B cell then produces more antibodies, which help target the pathogen for attack by neutrophils, macrophages, and dendritic cells.

T cells need help from the innate immune cells to become activated. A dendritic cell or macrophage that has ingested a pathogen will 'present' a small part of the antigen to a T cell. It's similar to a police dog sniffing a sample from a crime scene, then following the scent trail. This presentation happens in the lymph nodes or spleens. The T cell can do one of two jobs - either activate the immune response even more, or directly kill infected cells and cancer cells.

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