Your complete blood count (CBC) was probably measured if you have ever had blood removed for testing. CBC measures the percentage of erythrocytes (RBCs), the average shape and volume of RBCs, and the amount of hemoglobin, thrombocytes (platelets), and white blood cells (leukocytes) per cubic millimeter of blood. Whole blood comprises 55% plasma, 44% RBCs, and 1% leucocytes (WBCs) and thrombocytes.
The Lymphatic System
The majority of leukocytes (WBCs) reside outside of the blood. Most WBCs are in connective tissue and the lymphatic system. The lymphatic system consists of a network of vessels and organs. Most of the body’s extracellular fluid is interstitial fluid, which leaks out of cells and capillaries. Some of the interstitial fluid diffuses into lymph vessels, where it becomes lymph. (Plasma, interstitial fluid, and lymph are similar in composition, and their location within the body designates their name.) The lymph vessels have a series of filters called lymph nodes that team with white blood cells. The white blood cells kill the pathogens, which cleanses the lymph. After filtration, the lymph returns to the blood vessels and mixes with the blood plasma.
Lymph Nodes
Have you ever had a bump the size of a small marble appears behind your ear or in your armpit? If so, chances are it was a swollen lymph node. If touching was painful, your body was most likely fighting an infection. If it was not unpleasant to touch, you could’ve been recovering from an illness and, in rare cases, cancer. And sometimes, lymph nodes swell for an unknown reason.
So why do lymph nodes swell?
Most lymph nodes connect to more afferent lymph vessels (inlets) than efferent vessels (outlets). Under homeostatic conditions, this uneven distribution of lymph vessel attachments does not affect lymph flow through the lymph node. However, during infection, the number of lymphocytes increases, and the amount of infected lymph entering the lymph node far exceeds the amount of filtered lymph leaving the node. The combination of increased lymphocytes and fewer efferent vessels creates a bottleneck, and the lymph node swells. The lymph node’s swelling is analogous to a traffic accident that has shut down two lanes of a four-lane freeway during rush hour. The closure of the two lanes and the copious number of commuters will lead to a bottleneck (traffic).
Other Lymphatic Organs
The spleen! is located on the abdominal cavity’s left side, lateral to the stomach and pancreas, and superior to the intestines. It filters the blood of pathogens and dead cells.
The tonsils are open crypts located at the back of the throat. They contain white blood cells that attack pathogens and start the immune response.
The thymus gland is located above the heart, where T-lymphocytes, a type of white blood cell, mature and “learn” how to recognize specific pathogens. The thymus shrinks with age, one reason our immune systems get weaker in our twilight years (Team Edward!!!!).
Phagocytes and Cytotoxic Cells
Have you ever played Pac-Man?
Ok, stupid question. Anyway, Pac-Man’s shape and function are similar to that of a phagocyte. Phagocytes are WBCs that eat pathogens, cellular debris, and other unwanted stuff via phagocytosis, or as I call it, Pac-Mancytosis.
Ok, I’ll stop.
Phagocytosis is a type of active transport in which the cell surrounds its victim with its membrane and engulfs the pathogen. Inside the cell, the trapped pathogen resides in a small sack called a phagosome that will act as a digestive tract. A lysosome, an organelle, fuses with and empties its digestive enzymes into the phagosome. The phagosome is now a phagolysosome where the enzymes digest the pathogen into small pieces, killing it in the process. The phagolysosome merges with the cell membrane and dumps the dead pathogen’s remnants into the extracellular fluid.
Phagocytes can consume small pathogens like bacteria and viruses but cannot consume large cells such as cancer or viral-infected cells.
Here is an example of phagocytosis.
Fine. Here’s the real thing.
I’ll right. Here you go.
Cytotoxic cells kill cancer cells and cells infected by a virus by either inducing apoptosis or necrosis. The kill mechanism of cytotoxic cells is a protein message that tells cells to commit cell suicide (apoptosis) or cause the pathogenic cell to intake excessive water causing the infected cell to rupture.
Necrosis and Apoptosis
Like all life, cells spend a finite time in the living world. For example, the columnar cells that line the intestines live for a few days, while neurons and muscle cells remain active for decades. The different lifespans of the three hundred or so, cell types depend on a cell’s location and function. The short-lived cells that line our intestines live in a harsh, unforgiving environment of gastric acid, enzymes, and abrasive food. The skull and meninges protect the long-lived neurons in the hippocampus so we can learn things and store those memories for a lifetime.
A cell contains inflammatory chemicals in its cytoplasm. Inflammation is a product of the immune response, but too much inflammation or inflammation in the wrong location can lead to injury or disease. Dying cells reduce the release of inflammatory chemicals by initiating their “self-destruct button,” telling the cells to commit apoptosis.
Apoptosis refers to controlled cell death. Apoptotic cells do not explode but will break apart into little membrane-bound sacs known as blebs. The blebs trap the inflammatory chemicals within them, which prevents collateral damage.
Pathogens can also induce apoptosis, but necrosis is the more likely outcome. Necrosis is when a cell dies due to injury or by a pathogen. Necrotic cells begin blebbing, but the cell membrane ruptures before the blebs develop. The release of inflammatory cellular proteins into the extracellular fluid leads to inflammation. For example, when you sustain an injury, the site of the injury usually swells. The swelling primarily occurs due to cellular necrosis.
Granular Leukocytes
Granular leukocytes look like they have grains of sand in their cytoplasm called granules. The granules consist of anti-pathogen proteins and inflammatory proteins. Table 1 below explains the anatomy and physiology of granular leukocytes.
Agranular Leukocytes
Agranular leukocytes are a more diverse group of WBCs that do not have granules in their cytoplasm. Table 2 below explains the anatomy and physiology of monocytes, and table 3 explains the anatomy and physiology of lymphocytes.
