Organs of Immune System

To understand how the immune system works, you have to have fair idea of all the major players in our system and by players, of course I mean, the cells and organs in our body. There are different cell types involved in our immune system, each with its own unique job. These cells communicate among each other through a complicate pathway.

As Immunity topic is very vast, it can not be covered in a single blog post, from now on, there is going to be series of blogpost. Now let’s get straight into it.

Our Immune Organs

Different organs in our body produce the immune system cells. The cells then move around as needed via blood vessels and lymphatics and get stored in other organs. Here are some basics organs involved in our immune system.

Bone Marrow

Inside the firm outer core of bones lies the bone marrow, a spongy layer where blood cells are born. Most of this spongy layer is found in the flat bones, like the pelvis, skull, and sternum, as well as the end portions of the long bones of the arms and legs. In infants, most most of this bone marrow is red bone marrow, in which lots of cell transformation takes place. As you age, some of this red bone marrow gets replaced with fat and becomes yellow bone marrow. In time of emergency with significant blood loss, yellow marrow can convert to red marrow to help replace lost red cells, lymphocytes, and other blood cells.

In a process called hematopoiesis, stem cells are converted to red cells, lymphocytes, and other blood cells. B lymphocytes (I will talk about this in next blog) undergo some maturation in the bone marrow so that they are ready to perform their job when needed, other cells leave the bone marrow still in an immature state to morph into mature cells elsewhere.


The thymus is an organ found in the chest, directly above the heart. It’s the site where thymocyte mature into T cells. In infants, the thymus is large, but it gradually shrinks with age. Immature thymocytes move from the bone marrow to the thymus, where they interact with MHC (major histocompatibility complex). Here, they are shown different antigens, which are compounds that may or may not be a problem for the organism. If the T cell (I will talk about this next blog) react to antigens that are part of the organism itself, they get destroyed (otherwise, they may trigger an autoimmune reaction). if they react to only antigens that are not part of the organism, then they get to hang around. However, if they fail to respond to those things that are not part of the organism, they are also destroyed, because they are not helpful as part of the immune response. So there is active selection of both T cells that fail to work as well as those that react to antigens that are part of self.

After this process has occurred, the mature, now immunocompetent cells go out into the bloodstream to look for invaders.


The spleen is a smallish organ located in the upper left part of the abdomen. It acts mostly as a blood filter, gathering old red cells to destroy them, as well as recycling and processing antigens brought there by B cells, macrophages, and other cells of the immune system. It stores red cells for times of emergency and can also store platelets and lymphocytes for future use. Although it has an important function in the adult, the spleen is nonessential. If, for example, it gets damaged in an accident and has to be removed, its removal has no long- term effect on that person’s effect on that person’s immune function later in life.

Lymph nodes

Lymph nodes are small organs that are distributed throughout the body and connected by lymphatics vessels, channels through which lymph fluid flows. The lymph nodes act as filters of the lymph as well as housing for B cells, T cells, and macrophages. When an invading organism is encountered in the body, dendrites- phagocytes cells, grab the organism and take it into the lymph nodes. Here, lymphocytes make antibodies that hen go out into the bloodstream to fight against whatever the invader is. If there is a large infection and a lot of dendrites are bringing antigens back at once, the lymph node can become swollen, which is common in everything from colds to cancers.

Lymph nodes are scattered throughout the body but tend to be concentrated along areas where your own tissue has the most contact with organisms that are not you. The specific types of B and T cells that congregate in specific lymph nodes are determined by the most common invaders in that part of the body. The presence of these B and T cells saves a lot of time and trouble for your immune system, because it already has the right staff to perform the job and doesn’t have to wait for replacements.

How they work together

Structurally, the flow goes like this: cells of the immune system get formed primarily in the bone marrow. Then they migrate to the thymus for differentiation or to lymph nodes (or the spleen) to hang out and look for work. Due to the redundancy of the system, losing your spleen or some lymph nodes doesn’t have any long- term effect on your immune function.

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