Secondary Lympoid Organs: Lymph Nodes - Online Tutor, Practice Problems & Exam Prep

Lymph nodes are small, bean-shaped structures crucial for filtering lymph and trapping pathogens, with about 400 to 700 present in the human body. They consist of an outer cortex, rich in B lymphocytes, and an inner medulla containing both B and T lymphocytes. Lymph flows through afferent vessels into sinuses, allowing for immune interactions before exiting via efferent vessels. This slow lymph flow optimizes immune responses, highlighting the importance of lymph nodes in the immune system's activation and function.

Introduction to Lymph Nodes

In this video, we're going to begin our lesson on secondary lymphoid organs with an introduction to lymph nodes. Now lymph nodes are small bean-shaped structures, which is why we're showing you some images of beans down below so that you can compare them to the typical structure of a lymph node that we have right here. And notice that both of them have an indentation that you can see right here. This indentation in the lymph node is called the hilum. Now, these lymph nodes are found all along lymphatic vessels throughout practically the entire body, from your hands and your feet up through your arms and legs, into your torso region, and even up into your head. However, larger and more prominent lymph nodes are strategically located in regions of the body where lymphatic vessels tend to merge to form lymphatic trunks. And those are going to be the regions that we have indicated here in this image on the right-hand side, which are the cervical region or the cervical lymph nodes, found here in the neck region, the axillary lymph nodes, or the axillary region, which is the armpit or under the arm, and the inguinal lymph nodes or the inguinal region, which is this region where your thighs meet the abdomen.

Now, the primary function of these lymph nodes is to filter the lymph of harmful pathogens and foreign substances, and these lymph nodes are incredibly good at filtering the lymph. In fact, it's estimated that about 99% of all the pathogens and foreign substances that arrive at a lymph node are trapped within the lymph node and filtered from the lymph before the lymph is returned back to the bloodstream. So that's pretty remarkable, and it goes to show how important these lymph nodes are to our immunity. Now the amount, size, and shape of lymph nodes actually varies drastically from individual to individual. In terms of the amount, we can't quite put an exact number on it. However, typically, there are hundreds of lymph nodes in a single person. In fact, it's estimated that there can be anywhere from about 400 up to about 700 lymph nodes in a single person, which makes lymph nodes the most numerous lymphoid organ by far. Again, the size can also vary anywhere from about just a few millimeters in diameter up to over an inch in diameter. And although the shape is typically bean-shaped, the shape can also vary. But not only does the amount, size, and shape of lymph nodes vary from individual to individual, but it is also dynamic within each of us. The amount, size, and shape of lymph nodes can change within each of us depending on the specific situation and specific factors such as, for example, infection, inflammation, and immune activity. It's well known that when you do get sick, your lymph nodes tend to swell up and enlarge, and you can often feel them in your neck region when you are sick.

Because lymph nodes are secondary lymphoid organs, it's no surprise that each lymph node contains lots of immunocompetent lymphocytes, such as T and B lymphocytes, or T and B cells. And although the primary function of lymph nodes is to filter the lymph and trap pathogens, lymph nodes are also critical for immune system activation, essentially stimulating the immune system to generate a response. This is what makes these lymph nodes secondary lymphoid organs. Now, later in our course, we're going to talk a lot more details about immunity and a lot of these specific cells, and their roles. However, recall from previous lesson videos that dendritic cells are cells that are able to capture antigens in distant tissues, and they can actually transport those antigens to nearby lymph nodes and present those antigens to lymphocytes to activate the lymphocytes. Those activated lymphocytes could either carry out an immune response right then and there in that lymph node, or the activated lymphocytes may actually enter into the bloodstream and migrate to that distant site of infection to generate the immune response in that distant site of infection. Again, these are ideas that we'll get to talk more about later in our course when we're talking about immunity. But this here concludes our brief introduction to lymph nodes, and as we move forward, we'll get to talk more about the structure of the lymph node. So I'll see you all in our next video.

Secondary Lympoid Organs: Lymph Nodes Example 1

So here we have an example problem that asks, which of the following is not a function of lymph nodes? And we've got these 4 potential answer options down below. And option A says filtration of lymph or filtering out pathogens from the lymph, or in other words, removing disease-causing agents from the lymph. And, of course, this is definitely a function of lymph nodes. It's probably the main function of lymph nodes, if you will. And so we can eliminate answer option A because we are looking for the option that is not a function of lymph nodes.

Now option B says, providing a site for lymphocytes to mature. Now on first glance, this might seem like a function of lymph nodes. However, recall that lymphocytes mature in the primary lymphoid organs, which are the red bone marrow for B cells and the thymus for T cells. And so lymph nodes are not primary lymphoid organs; they are secondary lymphoid organs, which means that lymph nodes already have mature lymphocytes, and the lymph nodes do not provide a site for lymphocytes to mature, which means that answer option B is not a function of lymph nodes. So we can indicate that answer option B is the correct answer that we are looking for.

Now just to be sure, let's check option C and D. Option C says initiating the immune response when a foreign antigen is detected. And, of course, we know that this is also a function of lymph nodes, so we can cross off option C. And then option D says, these are all functions of lymph nodes, but, again, we've already identified option B as not a function of lymph nodes, so we can also cross off option D. And, again, option B is the correct answer to this example problem. That concludes this example, and I'll see you all in our next video.

In which of the following areas are lymph nodes usually highly concentrated?

• A. Feet.
• B. Chest.
• C. Hands.
• D. Neck.

Lymph Node Structure

In this video, we're going to talk about lymph node structure and how lymph actually flows through that structure, which are both critically important to the functions of lymph nodes. So notice down below, we've got this diagram representing the typical structure of a lymph node. And externally, these lymph nodes are surrounded by a capsule made of dense irregular connective tissue, which provides structural integrity to the lymph node and protects the delicate insides of the lymph node. So notice that down below in the diagram, the capsule is being labeled right over here. But again, it is going to surround the entire exterior of the lymph node as you can see here.

Now internally, lymph nodes are divided into 2 interior regions, which are the outer cortex and the inner medulla. And to make these two regions more clearly visible, we're going to highlight them in specific colors. So the outer cortex, we're going to highlight in this light blue color. So notice that down below in this diagram, everything that I'm highlighting in this light blue color represents the outer cortex of the lymph node, which notice resembles the shape of the letter C. And so you can think that the letter C in the outer cortex is for the shape that it resembles in many of these lymph nodes.

Now you can also see that the outer cortex makes up about 80-ish percent of the lymph node, and it encircles almost the entire lymph node, all except the hilum, which recall is this indentation that you can see here in the lymph node. Now the inner medulla, we will highlight in this hot pink color. And, of course, that's going to be this inner region that you can see right here, and it does lie deeper to the outer cortex, but the inner medulla does extend to the surface of the lymph node at the hilum.

Now this region that you can see that I'm highlighting here in this orangish color is sometimes referred to as the paracortex, since it lies in between the outer cortex and the inner medulla. Now what's important to note is that lymphocytes, such as B and T lymphocytes or B and T cells, are strategically located within the lymph nodes to optimize their functions. And so what you should note is that the outer cortex of lymph nodes has lymphoid follicles, which recall from previous lesson videos are also sometimes referred to as lymphoid nodules and consist of densely packed spheres of lymphoid tissues. And they often have germinal centers with populations of dividing B cells. And so the outer cortex of lymph nodes has those lymphoid follicles with mostly populations of B cells or B lymphocytes.

Now on the other hand, the inner medulla has lymphoid tissue that we refer to as medullary cords, and those medullary cords have populations of both B cells and T cells as well. So if we take a look at the diagram down below, notice on the far left, we're showing you the outer cortex, which is where the lymphoid follicles or the lymphoid nodules can be found. And, again, recall from previous lesson videos that these are going to be these densely packed spheres of lymphoid tissue that can have lighter staining germinal centers, which is where the populations of B cells are going to be actively dividing. And you can see that there are plenty of these lymphoid follicles in the outer cortex of each of these lymph nodes.

Now, you can also see in this diagram the inner medulla and the medullary cords, which again are going to be the lymphoid tissues in the medulla that contain populations of both B cells and T cells as well. Now the trabeculae are going to be inward capsular extensions that partially divide the outer cortex into compartments, and it also helps to redirect and direct the flow of lymph through the lymph node, which is critically important. So if we take a look down below at the diagram, you can see that one of the trabeculae are being labeled right over here, and, again, it's an inward extension of the capsule. And you can see plenty of these trabeculae all throughout, and, again, these trabeculae are breaking up the outer cortex into these compartments that we can see here.

Now the lymph nodes also contain what are known as sinuses, and these sinuses are series of lymph carrying channels or tunnels that have these branching reticular fibers and lymphoid cells as well. And so the lymph will actually flow through all of these different sinuses, and as they flow through the sinuses, they are going to be filtered of pathogens and foreign substances. So let's take a closer look at this diagram down below. And what you'll notice is that we've got these lymphatic vessels over here on the left hand side, and these lymphatic vessels are specifically referred to as afferent lymphatic vessels, spelled with the letter A. And this is because this is how lymph is going to arrive at the lymph node, and so you can think that the letter A in afferent lymphatic vessels is for the letter A in arriving at the lymph node.

And so notice that inside of these afferent lymphatic vessels over here, we've got these black arrows representing the arrival of lymph at the lymph node. Now on the opposite side of the lymph node over here at the hilum, we've got these other lymphatic vessels, which are specifically referred to as efferent lymphatic vessels, spelled with the letter E. And this is because this is how lymph is going to exit the lymph node. And so you can think that the letter E in efferent lymphatic vessels is for the letter E in exiting the lymph node.

Now what's really important to note is that there are more afferent lymphatic vessels and fewer efferent lymphatic vessels, which means that the rate of lymph arrival is significantly greater than the rate of lymph exiting the lymph node, which ultimately means that the rate of lymph flow through the lymph node is going to be relatively slow. And that slow rate of lymph flow is critically important because it optimizes immune interactions, which is critically important to the function of the lymph nodes.

Now, notice over here on the right in this box, we've got the lymph flow through the lymph node structures, and it's starting here at the top and making its way downwards. And so, of course, we know already that lymph is going to arrive at the lymph node via afferent lymphatic vessels, and from there, the lymph is going to enter into a series of sinuses, as you can see here. And again, recall that these sinuses are really just a series of lymph carrying channels or tunnels, and the first sinus that the lymph will enter into is the subcapsular sinus, which we have color-coded here in a yellow color. And as its name implies, the subcapsular sinus is going to be immediately beneath the capsule, and that's because the root sub means below and subcapsular means below the capsule. And so again, you can see that the sinus immediately beneath the capsule is highlighted in that yellow color for the subcapsular sinus.

Then from the subcapsular sinus, lymph will enter into the trabecular sinus, which we have color-coded in this blue color over here. And the trabecular sinus, as its name implies, is going to be parallel to the trabecula, and the subcapsular and trabecular sinuses are going to be found in the outer cortex of the lymph node. Now, from the trabecular sinus, the lymph is going to enter into the medullary sinus, which is going to be found in the medulla, and notice that the medullary sinus is going to be this hot pink color that you can see over here.

Now, from the medullary sinus, the lymph is going to enter into the efferent lymphatic vessels and exit the lymph node. And so you can see that these black arrows throughout this diagram show you the flow of lymph through the lymph node. And as lymph is flowing through the lymph node, the lymph is able to trickle and diffuse into other areas of the lymph nodes such as the lymphoid follicles and the medullary sinuses. And, of course, that lymph is going to be carrying potentially pathogens and foreign substances that can ultimately activate these immune cells. And those activated immune cells can either carry out an immune response right then and there inside of the lymph node, or those activated immune cells can actually enter into the bloodstream directly.

So notice that we've got an artery coming in here and a pair of capillaries and a vein coming out, and so the activated lymphocytes can actually make their way directly into the bloodstream and migrate to the site of infection and carry out an immune response at that distant site of infection. Now, the last note that I'll leave you all off with is that although in previous lesson videos, we said that all of the lymph is going to be returned to the bloodstream at the veins near the heart, this is going to be true, but it's not all of the lymph. It's the vast majority. So there is a very, very small percentage of lymph and a percentage of activated lymphocytes that can actually enter the bloodstream at each lymph node. And so, you can see here that the bloodstream is going to be closely connected with these lymph nodes. So it's an opportunity for some cleansed lymph to enter back into the bloodstream. So this here concludes our brief lesson on the lymph node structure, and we'll be able to get a lot of practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

Secondary Lympoid Organs: Lymph Nodes Example 2

So here we have an example problem that asks, which of the following features are present in both the thymus and in lymph nodes? And we've got these 4 potential answer options down below, which say medulla, sinuses, lymphoid nodules, or germinal centers. Now recall from our previous lesson videos that the thymus is somewhat unique amongst the other lymphoid organs and that it lacks lymphoid nodules or lymphoid follicles. And so option c is not going to be present in the thymus. So for that reason, we can eliminate answer option c. And recall that the germinal centers are the central areas within lymphoid nodules or lymphoid follicles that have populations of B cells that are actively proliferating or dividing. And so because the lymphoid nodules are missing from the thymus, so are the germinal centers. So we can eliminate answer option d as well. So now we're between either option a or option b. And recall that option b, the sinuses are present in the lymph nodes since these are going to be channels that carry lymph. However, the thymus does not have sinuses like the lymph nodes do. So for that reason, we can eliminate answer option b. And, of course, this leaves answer option a, which is the correct answer. Recall that the thymus has an outer cortex and an inner medulla in every lobule, and the lymph nodes also have an outer cortex and an inner medulla. So the medulla is found in both the thymus and the lymph nodes, although they serve different functions in these organs. The medulla is still a feature in each of these organs. So option a is the correct answer to this example problem. That concludes this example, and I'll see you all in our next video.

Lymph enters the convex side of a lymph node via ___________ lymphatic vessels. It moves through the _________ & then into the ____________. Lymph exits the node at its hilum, via ___________ lymphatic vessels.

• A. Efferent; cortex; medulla; afferent.
• B. Efferent; medulla; cortex; afferent.
• C. Afferent; cortex; medulla; efferent.
• D. Afferent; medulla; cortex; efferent.

Secondary Lympoid Organs: Lymph Nodes Example 3

So here we have an example problem that says to correctly label the indicated areas of the lymph node down below. Now over here on the left-hand side, this first label is labeling this lymphatic vessel, which is an afferent lymphatic vessel, which recall allows for lymph to arrive at the lymph node, and so you can think that the 'a' in afferent is for the 'a' in arrive. Now, on the other side over here, this label is labeling this lymphatic vessel, which is an efferent lymphatic vessel. And recall that the efferent lymphatic vessel will allow lymph to exit the lymph nodes. So you can think that the 'e' in efferent is for the 'e' in exit, and these efferent lymphatic vessels are present at the hilum, which is this indentation here that you can see. Now, one thing to notice here is that there are a lot more afferent lymphatic vessels than there are efferent lymphatic vessels. And what this means is that the flow of lymph is going to be slowed down in the lymph node, which is ideal for optimizing immune interactions. Now the next label that you can see over here on the far left is labeling this layer of dense irregular connective tissue that is encapsulating and surrounding the lymph node, and that is going to be the capsule. Now over here, this label is labeling one of the inward extensions of the capsule, and that is going to be the trabecula. And so, really, this here concludes this example problem, so I'll see you all in our next video.

Which of the following options correctly represents the flow of lymph through a lymph node?

• A. Afferent vessel > subcapsular sinus > trabecular sinus > medullary sinus > efferent vessel.
• B. Afferent vessel > trabecular sinus > subcapsular sinus > medullary sinus > efferent vessel.
• C. Efferent vessel > subcapsular sinus > trabecular sinus > medullary sinus > afferent vessel.
• D. Afferent vessel > subcapsular sinus > medullary sinus > trabecular sinus > efferent vessel.

Here’s what students ask on this topic:

What are the primary functions of lymph nodes in the human body?

Lymph nodes are crucial for filtering lymph and trapping pathogens. They contain immunocompetent lymphocytes, such as T and B cells, which help in immune system activation. Lymph nodes filter about 99% of pathogens and foreign substances from the lymph before it returns to the bloodstream. This filtration process is essential for maintaining immunity. Additionally, lymph nodes stimulate the immune system to generate a response, either locally within the lymph node or by sending activated lymphocytes to distant infection sites. This dual role in filtration and immune activation underscores their importance in the body's defense mechanisms.

How does lymph flow through a lymph node?

Lymph enters a lymph node through afferent lymphatic vessels and flows into a series of sinuses, starting with the subcapsular sinus beneath the capsule. It then moves into the trabecular sinus in the outer cortex and finally into the medullary sinus in the medulla. The lymph exits the lymph node via efferent lymphatic vessels at the hilum. The slow flow of lymph through these sinuses allows for optimal immune interactions, enabling lymphocytes to filter pathogens and foreign substances effectively. This slow rate is crucial for the lymph node's function in immune system activation and pathogen filtration.

What are the structural components of a lymph node?

A lymph node is surrounded by a capsule made of dense irregular connective tissue, providing structural integrity. Internally, it is divided into two main regions: the outer cortex and the inner medulla. The outer cortex contains lymphoid follicles rich in B lymphocytes, while the inner medulla has medullary cords containing both B and T lymphocytes. Trabeculae, inward extensions of the capsule, partially divide the outer cortex into compartments and help direct lymph flow. Sinuses, which are lymph-carrying channels, facilitate the filtration of lymph as it flows through the node.

Why do lymph nodes swell during an infection?

Lymph nodes swell during an infection due to the increased activity of lymphocytes and other immune cells within the node. When pathogens are detected, lymphocytes proliferate to mount an immune response, causing the lymph node to enlarge. This swelling is a sign that the immune system is actively fighting off the infection. The increased number of immune cells and the accumulation of filtered pathogens contribute to the noticeable swelling, particularly in regions like the neck, armpits, and groin, where larger lymph nodes are located.

What is the significance of the hilum in a lymph node?

The hilum is an indentation on the lymph node where efferent lymphatic vessels exit, and blood vessels enter and leave the node. It is a critical structure because it serves as the exit point for filtered lymph, which then re-enters the bloodstream. The hilum also allows for the entry and exit of blood vessels, facilitating the transport of activated lymphocytes into the bloodstream. This connection between the lymphatic and circulatory systems is essential for the immune response, enabling lymphocytes to reach distant sites of infection and carry out immune functions.