What does the spleen do

Overview

The spleen is a vital lymphoid organ located on the left side of the abdomen, positioned beneath the rib cage between the left kidney and the stomach. It is roughly the size of a clenched fist, measuring approximately 5-11 centimeters in length and weighing between 75 and 200 grams in adults, though this can vary based on individual physiology and health status. Despite being one of the lesser-known organs, the spleen performs essential functions related to blood filtration, immune response, and blood storage that are critical for maintaining overall health and protecting the body from pathogens.

Primary Functions of the Spleen

The spleen has three major functions in the human body, each contributing to overall health and immunity. First, it acts as a sophisticated blood filter, processing approximately 1 liter of blood every minute through specialized structures called splenic sinusoids. During this filtering process, the spleen removes pathogens, damaged red blood cells, and cellular debris from circulation. Studies have shown that the spleen removes approximately 90% of bacteria and pathogens from the bloodstream, making it one of the most effective immune organs in the body.

Blood Filtration and Cell Removal: The spleen contains two types of tissue—red pulp and white pulp—that work together to filter blood and generate immune responses. The red pulp contains specialized cells called macrophages that engulf and destroy pathogens, damaged red blood cells, and other harmful particles. These macrophages can eliminate approximately 2 million red blood cells per second, preventing the accumulation of defective cells that could impair oxygen transport. The spleen's filtering capacity allows it to process the approximately 24,000 liters of blood that circulate through the human body daily.

Immune Cell Production: The white pulp of the spleen contains lymphoid tissue that produces and stores white blood cells, including T lymphocytes and B lymphocytes. The spleen contains approximately 75% of all white blood cells in the body during resting conditions, making it a critical reservoir of immune cells. When the body faces an infection or immune challenge, the spleen can rapidly mobilize these cells to mount an immune response. The spleen produces antibodies that help neutralize pathogens and provides memory cells that recognize previously encountered infections within 24-48 hours.

Blood Storage and Emergency Release: The spleen serves as a blood reservoir, storing approximately 250 milliliters (8-9 fluid ounces) of blood through specialized storage mechanisms. This stored blood can be released into circulation during emergencies such as severe hemorrhage, intense exercise, or high-altitude conditions where the body needs additional oxygen-carrying capacity. During intense exercise or fight-or-flight responses, the spleen can contract and release up to 50-60% of its stored blood in less than 60 seconds, increasing the blood volume available to muscles by approximately 10-15%. This emergency blood release function is particularly important in situations where rapid oxygen delivery is critical for survival.

How the Spleen Works in the Immune System

The spleen's role in the immune system extends beyond simple filtration. It coordinates immune responses by presenting antigen information to lymphocytes, essentially training immune cells to recognize and respond to specific pathogens. When an infection occurs, the spleen can increase in size by up to 2-3 times its normal volume within 24-72 hours as it ramps up immune cell production and activation. This enlargement, called splenomegaly, is actually a sign that the spleen is actively fighting an infection.

The spleen also maintains the memory of past infections. B cells in the spleen produce long-lived plasma cells that continue to produce antibodies for years or even a lifetime after an infection or vaccination. This is why vaccination provides long-term immunity—the spleen essentially remembers the pathogen and produces antibodies on demand. This immunological memory is so effective that the spleen can generate a robust immune response upon re-exposure to a previously encountered pathogen within just 48-72 hours.

Blood Cells and Spleen Functions

The spleen plays a crucial role in red blood cell management. In addition to removing damaged or old red blood cells (which have a lifespan of approximately 120 days), the spleen also plays a role in filtering pathogens that infect red blood cells, such as malaria parasites. During malaria infection, the spleen can increase its filtering capacity by up to 10 times, becoming significantly enlarged as it attempts to remove infected cells from circulation.

For white blood cells, the spleen serves as both a production facility and a quality control checkpoint. It filters out dysfunctional or damaged white blood cells and produces new ones in response to immune challenges. The spleen also functions as a transit point where white blood cells can interact with antigens and receive activation signals before they circulate to other parts of the body to mount immune responses.

Common Misconceptions About the Spleen

Misconception 1: The spleen is not essential for survival. While it's true that people can survive without a spleen, this is not because the spleen is unimportant—rather, because the liver and other lymphoid tissues can partially compensate for its loss. People without a spleen do experience increased susceptibility to infections, particularly from bacteria like Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis. Studies show that asplenic individuals have infection rates that are 50-70% higher than healthy individuals, and they face a 70-fold increased risk of overwhelming sepsis from pneumococcal infection. This is why people who have had their spleen removed must receive specific vaccinations and sometimes prophylactic antibiotics.

Misconception 2: An enlarged spleen always indicates disease. While splenomegaly can indicate serious conditions like leukemia, lymphoma, or cirrhosis, it can also be a normal physiological response to acute infections, particularly in children. Viral infections like mononucleosis can cause the spleen to enlarge by 2-3 times its normal size as part of the normal immune response. In athletes and people living at high altitudes, the spleen can become moderately enlarged as it mobilizes additional blood storage. The spleen size naturally varies by 25-40% between individuals based on age, genetics, and overall health status.

Misconception 3: The spleen is the only organ that filters blood. While the spleen is excellent at filtering blood, the kidneys also filter blood, though for a different purpose—removing waste products and regulating water and electrolyte balance. The liver also filters blood, removing toxins and pathogens. However, only the spleen performs the specialized function of filtering pathogens with 90% efficiency and storing blood for emergency use.

Practical Considerations and Health Implications

Understanding spleen function is important for maintaining health and recognizing when something might be wrong. Individuals with enlarged spleens should avoid contact sports and abdominal trauma, as the enlarged organ is at greater risk of rupture, which can cause life-threatening internal bleeding requiring emergency surgical removal. Athletes with acute viral infections are often advised to rest for 3-4 weeks to allow the spleen to return to normal size before resuming vigorous exercise.

For people who have had their spleen removed due to injury or disease, specific preventive measures are important. The CDC recommends vaccination against Streptococcus pneumoniae (pneumococcal vaccine), Haemophilus influenzae type b (Hib vaccine), and Neisseria meningitidis (meningococcal vaccine) at least 2 weeks before spleen removal, or as soon as possible after if removal was emergency. Studies show that asplenic individuals who receive these vaccinations have significantly better outcomes than those who do not. Additionally, prophylactic antibiotics (typically penicillin or amoxicillin) are often recommended for asplenic individuals to further reduce infection risk by 50-60%.

The spleen's role in blood storage is particularly relevant for athletes and people at high altitudes. Endurance athletes can train the spleen to increase its blood storage capacity through consistent high-intensity exercise over weeks and months. This adaptation allows for increased oxygen delivery during competition, which is why some athletes show performance improvements at sea level after training at altitude—the spleen has been trained to store and release blood more effectively. This physiological adaptation is one of the reasons elite endurance athletes often have slightly enlarged spleens measuring up to 40-50% larger than sedentary individuals.