Where do lymphocytes sleep, and other naive questions

[GreatBigBore]

Ok, I will never post another biology question to yahoo answers.

I'm watching Carl Sagan's Cosmos for the billionth and billionth time, and suddenly something other than stars got my attention: lymphocytes. I watched them destroying bacteria and I wondered: do they just destroy the bacteria, or do they actually eat and digest them? I mean does a lymphocyte use its prey's remains for food? Or does the corpse just become waste in the bloodstream to be removed by the kidneys or whatever? And if so, then what do lymphocytes eat? I mean, how do they get their power? For that matter, how do red blood cells get their power? They deliver fuel to the other cells of the body, right? But how do they get their own fuel? And where from? The intestine, I guess?

And then, where do lymphocytes go when they're not on duty? I think I read that the NKs are just running around loose all the time, but I think that the small lymphocytes have to be activated somehow. Does that just mean that they're everywhere, sleeping until some signal travels through the body? Or do they live in a central location?

I've heard hypotheses that propose that a huge fraction of the human body is not actually human at all. I wonder if lymphocytes are actually human cells, or symbionts. Or maybe only recently sucked into the human genome? Wait, not human. Vertebrate--lymphocytes are diagnostic of vertebrates, right? So maybe lymphocytes in primitive vertebrates might still be bacterial symbionts? Or is this all just nonsense?

 

[Ygggdrasil]

I watched them destroying bacteria and I wondered: do they just destroy the bacteria, or do they actually eat and digest them?

When phagocytes encounter a bacterium, they surround the bacterium in their own membrane and engulf the bacterium. Once the bacterium is encased in a membrane-bound compartment inside of the phagocyte, the cell can deliver digestive enzymes into the compartment containing the bacterium. These enzymes will break down the bacteria into its component molecules (sugars, amino acids, nucleotides, lipids, etc.). The phagocyte can then either harvest these molecules from the membrane-bound compartment or it can just dump them back into the surrounding media.

And if so, then what do lymphocytes eat? I mean, how do they get their power? For that matter, how do red blood cells get their power? They deliver fuel to the other cells of the body, right? But how do they get their own fuel? And where from? The intestine, I guess?

When we eat food, enzymes in the stomach break down food into its component molecules. In the intestine, these molecules are extracted and brought into the blood stream. Cells throughout the body then acquire nutrients simply by taking them out of the blood stream. The cells then have the machinery to extract energy from these nutrients or use them as building blocks to construct new cellular components. Specialized cells in the body will store excess nutrients during times of plenty (i.e. right after eating) and release them when levels of the nutrient in the blood are too low.

Red blood cells don't deliver fuel to the cells, they deliver oxygen, a substance required for cells to convert their fuel into energy. Most of the fuel (glucose) is a small molecule that can floats around in the blood stream until it gets taken up by a cell.

And then, where do lymphocytes go when they're not on duty? I think I read that the NKs are just running around loose all the time, but I think that the small lymphocytes have to be activated somehow. Does that just mean that they're everywhere, sleeping until some signal travels through the body? Or do they live in a central location?

I'm not entirely certain, but I think they're just floating around in the blood stream like cops on patrol. They are inactive until they happen to pass by a site of infection where other immune cells are sending out signals calling for help.

I've heard hypotheses that propose that a huge fraction of the human body is not actually human at all. I wonder if lymphocytes are actually human cells, or symbionts. Or maybe only recently sucked into the human genome? Wait, not human. Vertebrate--lymphocytes are diagnostic of vertebrates, right? So maybe lymphocytes in primitive vertebrates might still be bacterial symbionts? Or is this all just nonsense?

Lymphocytes contain the same DNA as every other cell in the body. Symbionts would be expected to have different DNA from the rest of the cells in the body (for example, mitochondrion are symbionts inside of all eukaryotic cells, and they have their own DNA). Therefore, because lymphocytes contain human DNA, they are human.

It is correct that a huge fraction of the human body is not human. For example, in your gut, there are more bacterial cells than there are human cells in your entire body.

 

[Olle]

do they just destroy the bacteria, or do they actually eat and digest them? I mean does a lymphocyte use its prey's remains for food? Or does the corpse just become waste in the bloodstream to be removed by the kidneys or whatever?

Well, there are two types of offensive lymphocytes: NKs, which attack pathogens (hostile bacteria, etc) using toxins, and B lymphocytes, which use antibodies. Neither actually eat the bacteria, though; as you say, they kill them, and the waste is removed through lymph or kidneys or some other filtering/cleaning system.

And if so, then what do lymphocytes eat? I mean, how do they get their power?

Like most cells in the body: through respiration - burning of sugars and other nutrients. And just like other cells they acquire these nutrients through the bloodstream; in fact, since they live directly in the blood, they can just help themselves.

For that matter, how do red blood cells get their power? They deliver fuel to the other cells of the body, right? But how do they get their own fuel? And where from? The intestine, I guess?

Red blood cells also burn sugar to produce energy. (Mammalian red blood cells have no mitochondria, though, so they can only perform glycolysis - the first, and rather inefficient, step of cellular respiration.) The fuel, again, is sugars and such nutrients, from the surrounding blood. Nutrients that have been acquired from food in the intestine, yes, and then transported into the bloodstream for delivery to the whole body.

And what red blood cells deliver is oxygen, which isn't really a fuel - the fuel is the sugar and fat and so on; the oxygen is needed to perform the chemical reactions within the cells' mitochondria that digest the nutrients and release energy. (Note that there is oxygen freely in the blood, too; the red blood cells are just there to greatly increase the efficiency of transport. Some very strange fish even get by without red blood cells.)

And then, where do lymphocytes go when they're not on duty? I think I read that the NKs are just running around loose all the time, but I think that the small lymphocytes have to be activated somehow. Does that just mean that they're everywhere, sleeping until some signal travels through the body? Or do they live in a central location?

Well... there are many types of immune cells, so I can't give a general answer, but I'm quite sure most of them spend all of their lives active (or ready for action, as it were) in the bloodstream. Some immune cells wander freely in other tissues, or move back and forth between blood and lungs or what have you (so the term "white blood cell" is more a matter of tradition than accuracy). I think there might be some immune cells that lie dormant in some place until needed - if so, probably in the lymph nodes or the thymus - but I don't have time to look this up, because I'm about to be late for a movie. =)

Generally, as regards the immune system, the modus operandi is to produce more cells as needed, rather than have inactive ones floating about.

I've heard hypotheses that propose that a huge fraction of the human body is not actually human at all. I wonder if lymphocytes are actually human cells, or symbionts. Or maybe only recently sucked into the human genome? Wait, not human. Vertebrate--lymphocytes are diagnostic of vertebrates, right? So maybe lymphocytes in primitive vertebrates might still be bacterial symbionts? Or is this all just nonsense?

It's largely nonsense, but there are grains of truth in there.

The human body is not fully human in two ways:
1) The number of bacterial cells in the body is far larger (between 100 and 1000 times) than the number of human cells. But bacteria are very tiny, so in terms of mass, you're mostly human anyway. I believe an adult human has about a kilogram or two of bacteria in them.
2) Some of the machinery in our cells evolved from bacteria that lived symbiotically inside ancient cells, way way back before plants and animals had evolved - mitochondria and chloroplasts are the most well-studied examples. But they're so integrated in our system by now that we can consider them part of us anyway. Look up endosymbiosis on Wikipedia or somesuch; it's absolutely fascinating.

As for lymphocytes: no, they're definitely a product of the vertebrate body, and related to the other cells in your body. But like all of your cells, they have ancient symbiotic ex-bacteria working inside them.

 

[AyazM]

Lymphocytes do not "eat and digest" microorganisms. Lymphocytes are of three types: NK Cells, B and T lymphocytes. B lymphocytes have nothing to do directly with bacteria. It's their secretions that target bacteria: antibodies. NK cells and T lymphocytes kill by inducing apoptosis not in bacteria, but in HUMAN cells infected with bacteria or viruses.

MACROPHAGES are the ones that "eat" and "digest" bacteria. This is better known as phagocytosis.

Every cell in human body gets the energy from ATP molecules. Some processes that make ATP are Glycolysis, Krebs' Cycle (GTP), and Oxidative Phosphorylation. What process is used depends upon the type of cell but that's how the energy derived. Red blood cells have the same mechanism. The "fuel" that is used can either be glucose, fat or proteins (Yes, even proteins yield energy and they ARE used for that purpose normally also!).

As to where lymphocytes go, that depends upon the type of the lymphocyte. B and T are in a dynamic state alternating between "rest" and movement in blood. They just keep wandering from Lymph node to lymph node, and spleen to blood back to spleen back to blood. I hope you get the idea of the fact that it's all very DYNAMIC. There is no permanent resting state. There is constant movement. NK Cells keep floating in blood. They have no special predilection for any anatomic structure such as Spleen or Lymph Node.

Yes you are right. The "human" part of your human body is especially under voluntary control of your brain. Organs like abdominal organs and your heart are not under control of your brain's voluntary system. They just keep working on their own as if they're not part of your body. For cellular purposes, your mitochondria resemble bacteria FAR MORE than the cell they reside in!

That's the easiest way accurate information can be provided. You're welcome to ask if you're still confused about it!

 

[GreatBigBore]

Lymphocytes do not "eat and digest" microorganisms...

That's the easiest way accurate information can be provided. You're welcome to ask if you're still confused about it!

Hell yeah! Thanks so much for this answer. It's going to take a while to absorb it all, and then I'll probably have more questions. Thanks a lot.

 

[Smachine]

To add on what's already been said, lymphocytes are found in the blood as already mentioned and there are many lymphoid organs that contain many lymphocytes (mostly B and T helper), like lymph nodes, spleen, tonsils, MALT (mucosa associated lymphoid tissue) which is mostly gastrointestinal tracts and respiratory systems connective tissue under the layer of epithelium. They are also in the lymph.