Cells of the Immune System: Agranulocytes - Online Tutor, Practice Problems & Exam Prep

Agranulocytes are a type of leukocyte characterized by the absence of visible cytoplasmic granules under a light microscope. They include monocytes, which can differentiate into macrophages and dendritic cells, and lymphocytes, which consist of T cells, B cells, and natural killer cells. Monocytes play a crucial role in phagocytosis and immune response, while lymphocytes are essential for adaptive immunity, responding specifically to antigens. Understanding these cells is vital for grasping the complexities of the immune system and its functions in health and disease.

Agranulocytes

So now that we've discussed the granulocytes in our previous lesson videos, in this video we're going to begin to discuss the agranulocytes, which includes cells of the innate immune system. Now the agranulocytes are white blood cells or leukocytes with cytoplasmic granules. However, unlike the granulocytes, the cytoplasmic granules of agranulocytes are not visible under a light microscope. And because those cytoplasmic granules are not visible under a light microscope, we refer to them as agranulocytes. Now really, there are 2 main types of agranulocytes that we have numbered down below. The first main type of agranulocytes are going to be the monocytes, and the second main type of granulocytes are going to be the lymphocytes. Now the monocytes are leukocytes or white blood cells that circulate in the blood and can further develop into either macrophages or dendritic cells.

And so if we take a look at our image down below, notice that we're showing you our map of the lesson on the cells of the immune system. And so notice at the very top, we have our hematopoietic stem cell, and this hematopoietic stem cell can differentiate into the common myeloid progenitor cell or the common lymphoid progenitor cell and give rise to all of these other different cell types that we have down below. Now notice the ones that are in gray are the ones that we are not focusing on because we've already discussed them in our previous lesson videos, And this includes the granulocytes, which we already talked about. So in this video, we're focusing specifically on these cells over here, which are once again the agranulocytes. So we can go ahead and label them here as the agranulocytes. And these agranulocytes again include the monocytes, which you see here, and these lymphocytes, which you see over here.

Now the monocytes, as we just discussed up above in the text, the monocytes are leukocytes or white blood cells that can circulate in the blood and further develop into either macrophages or dendritic cells. And so taking a look at the image down below, notice we have the monocytes here, and they can further differentiate into either macrophages or dendritic cells. And we'll get to talk more details about the monocytes, macrophages, and dendritic cells as we move forward in our course.

Now the lymphocytes, once again, include these cells that we see over here, which are the natural killer cells, T cells, and B cells. Now notice that the natural killer cells are part of innate immunity. However, the T cells and B cells are part of adaptive immunity, and that's why they have that little star symbol next to their name right up here. And so, what we are saying here is that the lymphocytes include leukocytes involved in adaptive immunity. And the ones that are involved in adaptive immunity specifically refer to the T cells and B cells. Now adaptive immunity is a topic that we'll get to talk a lot more about in separate videos much later in our course, and so, that's something to keep in mind.

Moving forward here in our course, we're going to talk more details about these agranulocytes, starting off with the monocytes, macrophages, and dendritic cells, and then moving on to the lymphocytes. And so this here concludes our brief introduction to the agranulocytes, and I'll see you all in our next video.

Monocytes

This video, we're going to begin talking about monocytes and the cells that derive from monocytes. Taking a look at our map of the lesson down below on the left-hand side, notice that we're focusing specifically on these monocytes and the cells that are derived from them, including the macrophages and the dendritic cells. These monocytes are capable of developing into one of 2 main types of phagocytic cells or cells that are capable of performing phagocytosis or cellular eating. These two main types of cells are the macrophages and the dendritic cells. The macrophages are, once again, phagocytic cells, meaning that they're capable of performing phagocytosis. They also serve as sentinel cells, which we call lookout or guard cells that act as scouts. These macrophages are capable of responding directly to an infection to help eliminate it but can also alert other host defenses as well, and that is really what makes them sentinel cells. There are many different types of macrophages found in different types of tissues, and we refer to these macrophages as resident macrophages. Resident macrophages, as their name implies, reside permanently in specific tissues, which means that they remain stationary in the tissues, and they don't really migrate around to other tissues. They reside in the tissues. These resident macrophages, because they reside in the tissues, will already be present in the tissues whenever those tissues get damaged and microbes invade those tissues. They'll already be present and ready to respond because they reside permanently in those specific tissues. Other macrophages we refer to as wandering macrophages, and as their name implies, they wander around or move around through the blood to other sites of infection. These wandering macrophages can be recruited to a site of infection. Again, they travel around or wander around through the blood, circulate through the blood, and they can leave the bloodstream to a site of infection when they are recruited.

Now, giant cells refer specifically to macrophages that have fused together, and the fusion of macrophages creates these giant cells. The giant cells, because they consist of fused macrophages, have increased destructive abilities. If we take a look at our image down below on the left-hand side, we're specifically focusing on the macrophages. Here we're showing you an image of the tissues, the skin here. Notice that the skin here has been damaged, that there's some kind of damage here, and there's some kind of sharp object that has penetrated into the skin, and microbes are now infecting this open wound. Notice that there are already some macrophages that are present in the tissues already, and we refer to these as resident macrophages because they reside. They are living in the tissues, and they're already present and ready to respond whenever any of our tissues get infected. Wandering macrophages, on the other hand, circulate through the bloodstream. As they circulate through the bloodstream, they can be recruited to a site of infection. The resident macrophages reside and remain stationary within a specific type of tissue, whereas the wandering macrophage can circulate through the blood and move to other locations and be recruited to sites of infection.

The second main type of cells that monocytes can develop into are going to be the dendritic cells. Dendritic cells also serve as phagocytes, but they are mainly going to be important as sentinel cells, which again are lookout or guard cells that act as scouts. Their job is to sense and detect the signs of microbes, and then they can alert other immune system cells and activate other immune system cells. These dendritic cells are sentinel cells with long appendages. They reside in tissues, and they are capable of alerting and initiating adaptive immunity and adaptive immune responses. They can alert and activate B cells and T cells. We'll be able to talk more about this process of dendritic cells alerting and initiating adaptive immunity later in our course when we're talking more about adaptive immunity. Ultimately here, what we're saying is that these dendritic cells can ingest pathogens in the tissues. They can ingest those pathogens, break them down, and then present pieces of those pathogens to cells of the adaptive immune system in order to activate those adaptive immune system cells. Once again, we'll get to talk more about this later in our course, but down below we're showing you a little image of these dendritic cells. These dendritic cells have these long appendages, as you see here, that project out. Here what we're showing you is a micrograph of what these dendritic cells can look like under a microscope. Notice that they have these long appendages that stick out. Here we're showing you the intestinal lumen. This is our intestines, for example, our large intestine. Some of the substances that we end up eating get broken down into antigens, and these antigens could be potentially from microbes that we have eaten. What you can see here is that the dendritic cells, which are down below, have these long appendages that can take little samples of the antigens in our intestine, and then they can ingest those antigens, break them down and process them, and then present them on their surface. So, notice that these little pieces of antigen are presented on their surface, and here's a second dendritic cell doing something similar. Then these dendritic cells can go on to activate other adaptive immune system cells like B cells and T cells, which once again, we'll get to talk more about that process later in our course when we're focusing on the adaptive immune system. But for now, this here concludes our brief lesson on the monocytes and how they can develop into macrophages and dendritic cells. We'll be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.

All of the following are major differences between macrophages & neutrophils except which of these answers?

• A. Macrophages can perform phagocytosis while neutrophils cannot.
• B. Macrophages kill microbes by 'eating' them. Neutrophils kill microbes with hydrolytic enzymes and/or phagocytosis.
• C. Neutrophils circulate the bloodstream while resident macrophages reside in tissues.
• D. Neutrophils are granulocytes while macrophages are agranulocytes.

Monocytes can differentiate into which types of immune cells?

• A. Basophils & macrophages.
• B. Neutrophils & dendritic cells.
• C. Macrophages & dendritic cells.
• D. Mast cells & macrophages.

Introduction to Lymphocytes

In this video, we're going to begin our introduction to lymphocytes. Taking a look at our map of the lesson down below, we have natural killer cells, T cells, and B cells. Notice that these lymphocytes are all derived from common lymphoid progenitor cells. The T cells and B cells are the only lymphocytes listed here that are part of adaptive immunity. However, the natural killer cells, although they are lymphocytes, are not part of adaptive immunity but are part of innate immunity. Notice here that we're saying these lymphocytes include leukocytes that are important for adaptive immunity. But really, we're focusing specifically on the T cells and B cells when talking about lymphocytes important for adaptive immunity.

Adaptive immunity is a topic that we'll talk a lot more about later in our course, and it will be covered in other videos. The two major groups of lymphocytes that are important in adaptive immunity are once again the T cells and B cells. These cells respond to very specific antigens of invading microbes, and this specificity is a characteristic of adaptive immunity. Not all lymphocytes are part of adaptive immunity. It's important to note that there is a group of lymphocytes, specifically the innate lymphoid cells (ILCs), that differ from the B and T cells. The B and T cells are part of adaptive immunity, but the innate lymphoid cells are part of innate immunity. The innate lymphoid cells are commonly abbreviated as ILCs, and they differ from the B and T cells due to a lack of specificity in antigen recognition. While T cells and B cells are very specific and part of adaptive immunity, the innate lymphoid cells lack specificity.

A classic example of ILCs, or innate lymphoid cells, are the natural killer cells, abbreviated as NK cells. These natural killer cells are innate lymphoid cells, and they kill a variety of different cell types, so they are not specific to one type of antigen. They are non-specific. Taking a look at this image over here, notice that we're focusing specifically on the lymphocytes. The lymphocytes include natural killer cells, which are innate lymphoid cells that are part of innate immunity, but it also includes the T cells and B cells, which are important for adaptive immunity and adaptive immune responses.

We'll be able to talk in much more detail about the T cells and B cells later in our course when discussing adaptive immune responses. We will also talk more about the lymphocytes at that time, including more details about the natural killer cells in those separate videos. For now, this concludes our brief introduction to lymphocytes, and we'll be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.

Why is a bone marrow transplant used to replace defective lymphocytes in a patient?

• A. Bone marrow creates common myeloid progenitor cells which differentiate into lymphocytes.
• B. Bone marrow creates monocytes which differentiate into lymphocytes.
• C. Bone marrow creates hematopoietic stem cells which differentiate into all immune cells including lymphocytes.

Which of the following immune cells is NOT a professional phagocyte?

• A. NK cell.
• B. Neutrophil.
• C. Macrophage.
• D. Dendritic cell.

Here’s what students ask on this topic:

What are agranulocytes and how do they differ from granulocytes?

Agranulocytes are a type of leukocyte (white blood cell) characterized by the absence of visible cytoplasmic granules under a light microscope. They include monocytes and lymphocytes. Monocytes can differentiate into macrophages and dendritic cells, while lymphocytes consist of T cells, B cells, and natural killer cells. In contrast, granulocytes have visible cytoplasmic granules and include neutrophils, eosinophils, and basophils. The primary difference lies in the visibility of these granules under a light microscope and their roles in the immune response. Agranulocytes are crucial for both innate and adaptive immunity, whereas granulocytes are mainly involved in the innate immune response.

What roles do monocytes play in the immune system?

Monocytes are a type of agranulocyte that circulate in the blood and can differentiate into macrophages and dendritic cells. They play a crucial role in phagocytosis, where they ingest and break down pathogens and cellular debris. Macrophages, derived from monocytes, act as sentinel cells, responding directly to infections and alerting other immune cells. Dendritic cells, also derived from monocytes, serve as antigen-presenting cells that activate T cells and initiate adaptive immune responses. Thus, monocytes are essential for both innate and adaptive immunity, helping to eliminate infections and activate other immune defenses.

What are the differences between T cells and B cells?

T cells and B cells are both lymphocytes involved in adaptive immunity, but they have distinct roles. T cells mature in the thymus and are primarily responsible for cell-mediated immunity. They can directly kill infected cells or help other immune cells. B cells mature in the bone marrow and are responsible for humoral immunity. They produce antibodies that bind to specific antigens, neutralizing pathogens or marking them for destruction by other immune cells. Both T cells and B cells are highly specific to particular antigens, a hallmark of adaptive immunity.

How do natural killer (NK) cells function in the immune system?

Natural killer (NK) cells are a type of lymphocyte that plays a crucial role in the innate immune system. Unlike T cells and B cells, NK cells do not require prior exposure to a specific antigen to act. They can recognize and kill a variety of infected or cancerous cells by detecting changes in the levels of certain surface molecules. NK cells release cytotoxic granules that induce apoptosis (programmed cell death) in the target cells. Their ability to respond quickly to a wide range of threats makes them an essential component of the body's first line of defense.

What is the role of dendritic cells in the immune system?

Dendritic cells are antigen-presenting cells derived from monocytes. They act as sentinel cells, detecting pathogens in tissues and ingesting them through phagocytosis. After processing the pathogens, dendritic cells present pieces of these antigens on their surface to T cells, thereby initiating adaptive immune responses. This antigen presentation is crucial for activating T cells and orchestrating a targeted immune response. Dendritic cells bridge the innate and adaptive immune systems, ensuring that the body can mount a specific and effective defense against pathogens.