Chapters
In this article, we will discuss the sequence of events that occurs during a primary immune response with reference to the roles of, macrophages, B-lymphocytes which includes plasma cells, and T-lymphocytes, limited to T-helper cells and T-killer cells. In addition to this, we will also explain the role of memory cells in the secondary immune response and long-term immunity. So, let us get started.
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1st lesson free!
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1st lesson free!
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Primary Immune Response
- Another type of white blood cell which plays a fundamental role in the specific immune response is Lymphocytes.
- Lymphocytes are smaller as compared to phagocytes and they contain a large nucleus that fills the majority of the cell.
- The two types of lymphocytes are:
- B-lymphocytes or B cells
- T-lymphocytes or T cells
Each of the above two types of lymphocytes has a distinct mode of action.
Macrophages
- Macrophages are long-lived cells that are larger as compared to neutrophils.
- Instead of remaining in the blood, macrophages travel into organs which include the liver, kidneys, lungs, spleen, and lymph nodes.
- Macrophages are produced in the bone marrow and move in the blood as monocytes, which then develop into macrophages as they leave the blood to settle in different organs as mentioned above.
- Macrophages play a vital role in starting an immune response
- Although they carry out phagocytosis in the same way as neutrophils, they do not completely destroy the pathogens. Instead, they cut the pathogens up, so that their antigens can be displayed
- Lymphocytes can then recognize these exposed pathogens
B-lymphocytes
- B-lymphocytes or B cells remain in the bone marrow before they mature. Once they are mature, they spread through the body. The greater concentration of B cells is in lymph nodes and spleen
- Our body produces a million types of B cells because once they mature the genes coding for antibodies are altered to code for various antibodies
- After they mature, each kind of B cell can create a single type of antibody molecule
- At this stage, the antibody molecules remain in the cell surface membrane which implies that they do not leave the B-lymphocyte cell
- A part of each antibody molecule creates a glycoprotein receptor that combines specifically with a single type of antigen
- A small number of B-lymphocytes with receptors complementary to that antigen are triggered to undergo division by mitosis when an antigen enters the body for the first time. This is referred to as clonal selection
- Because these clones undergo repeated division by mitosis, they result in the production of a large number of identical B-lymphocytes over a few weeks
- These B-lymphocytes then create the following two types of cells during an immune response:
- Some of the B-lymphocytes transform into the plasma cells that secrete a huge number of antibody molecules into the lymph, blood, or linings of the gut and lungs
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- The number of these plasma cells declines after many weeks which implies that they are short-lived. However, the antibodies they secrete remain in the blood for a longer time
- The other B-lymphocytes become the memory cells that keep circulating in the blood for a long time
- There is a slow response to a newly encountered antigen and it is referred to as a primary immune response
T-lymphocytes
- Immature T-lymphocytes mature in the thymus after leaving the bone marrow
- Mature T-lymphocytes have particular cell surface receptors which are known as T cell receptors
- The structure of these receptors is the same as antibodies and each receptor is specific to a single antigen
- T-lymphocytes are stimulated after they encounter their particular antigen that is presented by one of the host’s cell
- This antigen-presenting host cell can either be a body cell or a macrophage that is attacked by a pathogen and it has displayed the antigen on its cell surface membrane
- These stimulated T-lymphocytes undergo division by mitosis to increase their number. This is the same as the clonal selection or clonal expansion of B-lymphocytes
- These T-lymphocytes then differentiate into two primary types of T-cell:
- Helper T cells
- killer T cells
- Cytokines released by the helper T cells stimulate B-lymphocytes to divide and develop into antibody-secreting plasma cells. Few helper T cells secrete cytokines that trigger macrophages to boost their rates of phagocytosis.
- Killer T cells attach to the antigens on the cell surface membranes of infected cells and secrete harmful substances that not only kill the pathogen inside the body cell but also the cell itself
Role of Memory Cells in Long-Term Immunity
- B-lymphocytes create two types of cells: plasma cells and memory cells during an immune response
- Memory cells create the basis of immunological memory. These cells can survive for several years, and generally, they last a lifetime
- The immune response is of two types:
- Primary immune response (it responds to a newly encountered antigen)
- Secondary immune response (it responds to an antigen that has been encountered previously)
Primary Immune Response
- A small number of B-lymphocytes with receptors complementary to that antigen are triggered to undergo division by mitosis when an antigen enters the body for the first time. This is referred to as clonal selection
- Because these clones undergo repeated division by mitosis, they result in the production of large numbers of identical B-lymphocytes over a few weeks
- These B-lymphocytes then create the following two types of cells during an immune response:
- Some of the B-lymphocytes transform into the plasma cells that secrete a huge number of antibody molecules into the lymph, blood, or linings of the gut and lungs.
- The number of these plasma cells declines after many weeks which implies that they are short-lived. However, the antibodies they secrete remain in the blood for a longer time
- The other B-lymphocytes become the memory cells that keep circulating in the blood for a long time
- There is a slow response to a newly encountered antigen and it is referred to as a primary immune response
Secondary Immune Response
- If the same antigen is present in the body for the second time, the memory cells can recognize this antigen, divide rapidly and differentiate into plasma cells to produce more memory cells and antibodies
- This response is quite quick which implies that the infection can be destroyed and removed before the increase in the population of pathogen and the development of symptoms of the disease
- As compared to the primary immune response, the response to a previously encountered pathogen is quite rapid
- T-lymphocytes also play a fundamental role in the secondary immune response.
- They produce two primary cells after differentiating into the memory cells:
- Memory helper T cells
- Memory killer T cells
- Just like memory cells which are created from B-lymphocytes, these memory T cells stay in the body for a longer period
- If the body has the same antigen for the second time, these memory T cells become stimulated quite quickly
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