What the heck is the spleen?

Feb 21, 2020

By Jenna Beam

Let’s level with each other – everyone has a spleen, everyone knows that everyone has a spleen, but does anyone know what the spleen is for? 

Until a few years ago, I had no idea what the spleen was for. Was it like the appendix – a vestigial structure, leftover from millenia of evolution, but now useless? Who knew? Not me, that’s for sure. But I’m here to tell you that the spleen is actually one of the most important organs in the immune system.

Your immune system is your body’s defense system. It’s made up of immune organs, like bone marrow, the thymus, lymph nodes, and – you guessed it – the spleen. There are also lots of different kinds of immune cells. Your immune cells constantly circulate through your blood and through your immune organs looking out for anything that’s trying to invade. Basically, your immune cells are there to protect you from things that are “not you.” Typically, things that are “not you” are pathogens, like bacteria, viruses, or parasites. The parts of pathogens that cause an immune reaction are called antigens

Now that we’ve covered the immune system, let’s get back to the star of our show. Your spleen is a small, roughly fist-sized organ that sits behind your stomach. In general, the spleen is important for filtering your blood. Most of the spleen is made up of red pulp, where dead red blood cells go to be recycled. Another part of the spleen, the white pulp, is where the immune business happens. 

Immune cells constantly circulate through your blood, which means they inevitably also circulate through the spleen. If they happen to interact with antigen in the spleen, they kick defense mode into high gear. Special immune cells called B-cells start dividing at super speed and making antibodies against antigens that they saw in the spleen.There are also T-cells, another special type of immune cell, that can either help the B-cells make antibodies (helper T-cells) or can kill other cells that might be infected (killer T-cells).

Outside of B- and T-cells, all other kinds of immune cells circulate in and out of the spleen, making sure that anything that’s “not you” doesn’t get to take up residence in your body. These include some less specific (though still very important) cells like macrophages and dendritic cells, which are part of your innate immune system (watch the linked video to learn all about innate versus adaptive immunity). 

When antigen (gray rod with diamonds) and immune cells meet in the spleen, your immune cells kick it into high gear. B-cells (blue) start quickly dividing. They also make and secrete, or spit out, antibodies (Y shapes). Some T-cells can kill infected cells (killer T-cells) and some T-cells help B-cells make antibodies (helper T-cells).

So what the heck is the spleen? The spleen is a filter, a home for immune cells, and a really cool and important organ for keeping you healthy. 

If you want to read more about the immune system, check out this website! If video lessons are more your speed, check out this video from Harvard Medical School. 

Edited by Christian Agosto-Burgos and Lane Scher