Chapters
In this article, we will discuss the mode of action of phagocytes (macrophages and neutrophils). Moreover, we will also discuss antigens and state the difference between self-antigens and non-self antigens. But before proceeding to discuss the mode of phagocytes, first, let us define immunity.
What is Immunity?
Immunity refers to the ability to identify the intrusion of material unknown or foreign to the body and mobilize cell and cell products to eliminate that unknown material with the highest efficiency and effectiveness.
There are two types of immunity:
Naturally Acquired Immunity
Naturally acquired immunity can be either gained passively or actively.
- Passively acquired natural immunity
This can be acquired due to the transfer of antibodies from the mother to the fetus across the placenta. This is a short-term immunity because no memory cell is involved.
- Actively acquired natural immunity
It is acquired when the body is exposed to an infection and it produces its own antibodies to fight that infection, along with the memory cells. Due to the involvement of these memory cells, when the body is exposed to a similar infection, the response time decreases.
Artificially Acquired Immunity
Like naturally acquired immunity, artificially acquired immunity can also be achieved either passively or actively.
- Passively acquired artificial immunity
This immunity is also acquired for a shorter duration because no memory cells are produced. It is acquired by injecting ready-made antibodies into the body to fight certain infections like tetanus.
- Actively acquired artificial immunity
This type of immunity is acquired when the vaccine is given to a healthy body. As a response to the vaccine, the body stimulates to produce antibodies and memory cells against the vaccine. Hence, in this way, the body builds an immunity to the disease.
In the next section of the article, we will discuss the mode of action of phagocytes.
Origin and mode of action of Phagocytes
- Phagocytes refer to the white blood cells that are made non-stop in the bone marrow
- Before being distributed around the body in the blood, the phagocytes are stored in the bone marrow
- The primary function of phagocytes is to remove dead cells and intrusive microorganisms
- They are responsible for carrying out a non-specific immune response
Types of Phagocytes
There are two types of phagocytes:
- Neutrophils
- Macrophages
Both neutrophils and macrophages have their mode of action. Because both of these are phagocytes, hence they both carry out phagocytosis. Phagocytosis refers to the process of identifying and engulfing a pathogen. Each type of phagocyte utilizes a slightly different procedure to achieve this.
Neutrophils
- Neutrophils move around the body and leave the blood by pressing through capillary walls to watch the body tissues
- Neutrophils are released in large numbers from their stores during an infection.
- Neutrophils are short-lived cells
Mode of Action of Neutrophils
- Pathogens and the body cells under attack release chemicals. An example of a chemical released by the body cells under attack includes histamine. These chemicals attract neutrophils to the site where pathogens are present. This response to chemical stimuli is known as chemotaxis.
- Neutrophils travel toward the pathogens which may be covered in antibodies
- The antibodies act as another stimulus for the neutrophils prompting them to attack the pathogens. Remember that the neutrophils contain receptor proteins on their surfaces that identify antibody molecules and attach to them.
- When the neutrophils are attached to the pathogens, the cell surface membrane of a neutrophil stretches out and around the pathogen to engulf it and trap it within a phagocytic vacuole.
- This part of the process is referred to as endocytosis
- The digestive enzymes are then secreted by the neutrophil into the vacuole. These enzymes are released from lysosomes which merge with the phagocytic vacuole.
- The pathogen is destroyed because of this digestive enzyme
- The neutrophils die after killing and digesting the pathogens. The sign of a dead neutrophil includes pus.
Macrophages
- Macrophages are long-lived cells that are larger than neutrophils
- Instead of remaining in the blood, the macrophages move into the organs which include the liver, spleen, kidneys, lungs, and lymph nodes.
- Once produced in the bone marrow, the macrophages move in the blood as monocytes. The monocytes then develop into macrophages after leaving the blood to settle in different organs as mentioned above.
Mode of Action of Macrophages
- Macrophages play a critical role in starting an immune response
- Although they carry out the process of phagocytosis in the same way as neutrophils, the pathogens are not completely destroyed by them
- Macrophages cut up the pathogens to display the antigens of pathogens on their surface through a structure known as the major histocompatibility complex
- These displayed antigens can then be identified by lymphocytes. Lymphocytes are another type of white blood cell.
In the next section of the article, we will discuss antigens in detail.
Antigens: Self and Non-Self
An antigen refers to any substance that stimulates the production of antibodies when it is introduced into the tissue or blood. The majority of these cells have antigens in their cell surface membrane which play the role of makers enabling cells to identify each other.
Antigens are large complex molecules like proteins and glycoproteins. In response to the foreign antigens, the body usually produces antibodies.
Each cell in the human body contains markers that are used to distinguish it. Microorganisms whether pathogenic or non-pathogenic like bacteria and viruses, also contain their own unique markers. These markers are known as antigens and they enable the cell to cell recognition.
Antigens are present on cell surface membranes, bacterial cell walls, or the surfaces of viruses. Few glycolipids and glycoproteins on the outer surface of the cell surface membranes also play the role of antigens.
Self and Non self antigens
Antigens are either self antigens or non-self antigens.
Self Antigens
Antigens that are produced by the organism’s own body cells are referred to as self-antigens. Self antigens do not trigger an immune response.
Non-self Antigens
Antigens that are not produced by the organism’s own body cells are referred to as non-self antigens. Non self antigens trigger an immune response.
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