Virtual Medical Centre

Acquired Immune System (B Cells and T Cells)

• Introduction to the acquired immune system
• Activation of the acquired immune system
• Lymphocytes
  • T cells: Mediated immunity
  • B cells: Humoral immunity

Introduction to the acquired immune system

The human acquired immune system is responsible for the destruction of foreign particles once they have entered the body. Before it has seen a foreign particle, it is actually quite ignorant about how to destroy it. During the first exposure to an invader (which could be a virus, a bacteria or any unwanted particle), the acquired immune system must 'learn' how to attack and destroy the foreign particle. This means that it is not as good as the innate immune system for keeping out things that it has never encountered before. Once the acquired immune system has created a response, however, a protective response can be made more quickly and with greater force, allowing it to protect the body from harm.

The cells of the acquired immune system are mainly the B cells and the T cells, but there are also other important parts of the acquired immune system, such as the 'complement cascade' and the production of antibodies. The acquired immune system also plays the key role in the rejection of implanted tissue.  

Activation of the acquired immune system

Unlike the innate immune system, the acquired immune system needs to have seen a substance before in order to attack it effectively. This is because the way that the acquired immune system attacks a target is very specific and takes time to prepare.

All agents foreign to your body have unique patterns on their surfaces that allow the cells of the acquired immune system to detect them. When the cells of the aquired immune system detect these patterns, the agents are recognised as foreign, and the immune system can therefore mount an attack. Anything that the immune system can detect and attack is called an antigen.

The activation of the acquired immune system initially requires the help of other cells. The cells of the acquired immune system are coated in receptors. These are highly specific molecules designed to recognise certain substances. The receptors are so specific that each receptor can only recognise one substance and nothing else. There are many immune cells in the blood, each with its own different receptor. This means that the body can be protected against many different things.

Cells called macrophages (which means 'big eater') can speed up the process of activation. Macrophages are found in lots of places throughout the body, and eat anything that they do not recognise. After they have eaten something, the macrophages break it down into its basic proteins and present these to the immune cells. This causes a better, more accurate and more damaging response than the macrophages alone are capable of producing.

Lymphocytes

Lymphocytes are a type of white blood cell. There are two types of lymphocytes of the acquired immune system: T cells and B cells. There is a third type of lymphocyte known as natural killer (NK) cells, but these are a part of the innate immune system.

T cells: Mediated immunity

T cells account for about 80% of all lymphocytes. They are named T cells because they mature in the thymus, a gland found in the chest. There are three types of T cell lymphocytes:

• Cytotoxic T cells
• Suppressor T cells 
• Helper T cells

These cells all play a role in the direct destruction of problem cells in the body, such as cells infected with a virus, or cells with DNA damage (e.g. some cancer cells).

T cell production

T cells start out as stem cells (early types of cells that have not yet fully grown) and are produced by bone marrow. To mature, these stem cells move to the thymus, where they can stay for up to three weeks. About 99% of T cells do not make it to maturity. This is because the body is very selective about what T cells are produced so that they do not cause damage to the body's own cells. In the thymus, the T cells are given T cell receptors, of which there are several types. The type of receptor received determines what type of T cell it will be, what its role is, and which cells it can interact with.

T cell function

T cells function both through the release of substances into the blood, and by signalling B cells through contact. They have several different roles:

• Signalling for growth and activation of B cells
• Activation of cells that can 'eat' foreign substances
• Stimulation of cytotoxic T cells during a viral infection
• Signalling growth in cells, including other T cells, macrophages and eosinophils

T cell activation

T cells cannot detect foreign substances without assistance, and require a complex system to help them work. They need the help of cells called antigen presenting cells (APCs). These cells will eat the foreign substance, be it a bacteria, virus-infected cell or toxin, break it down, and present part of it to the T cell so that it can mount a response. APCs have a special type of molecule on their surface that allows them to communicate with helper T cells. Once a response is activated, lots and lots of T cells of different types are released into the blood stream. The released cells are responsible for the destruction of the foreign substance.

Helper T cells

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Helper T cells are by far the most common T cell. They make up more than three quarters of the T cell population. Helper T cells help the immune system in many different ways, and serve as a major regulator of virtually all immune functions in the body. They mainly act through the release of substances that help control the other parts of the immune system. These substances (called lymphokines) stimulate the other types of T cells to grow and attack. They also help B cells grow and mature into their active form. In acquired immunodeficiency syndrome (AIDS), there is a loss of helper T cells, leaving the body open to infection. Also, due to the influence of helper T cells on B cells, B cells may be inactive in cases where the T cells are damaged.

Cytotoxic T cells

Cytotoxic T cells are important in defending against virally infected cells, in the rejection of tissue grafts, and in the immune response to certain tumour types. Cytotoxic T cells require activated APCs, and rely on the presence of helper T cells.

Following activation by helper T cells, cytotoxic T cells prepare for the destruction of their target. Inside the cells, substances are formed which are incredibly dangerous to cells. They create a protein called perforin, named because it has the ability to 'perforate' infected cells by punching holes in them. Cytotoxic T cells can also release enzymes that destroy the cell structure.

To destroy a cell, a cytotoxic T cell first latches on to it, then releases the aforementioned substances directly onto the cell. Consequently, there is no damage to any other cell that happens to be nearby. The released substances cause the cell to self-destruct rather than explode. Thus, if there are any viral particles in the cell, they will be destroyed with the cell rather than be released to spread. After the cell is destroyed, the cytotoxic T cell can detach itself and leave to destroy other infected or otherwise damaged cells.

Suppressor T cells

Suppressor T cells are, as the name suggests, capable of suppressing the functions of both helper and cytotoxic T cells. It is believed that suppressor cells function as regulators of the other cells of the immune system, stopping them from causing excessive damage to the body's own tissues. It is probable that suppressor T cells play an important role in protecting against autoimmune attack.

B cells: Humoral immunity

B cells account for 10–15% of circulating lymphocytes. They are called B cells because they were first discovered to mature in the 'bursa of Fabricius' organ in birds. Humans no longer have this organ, and so B cell maturation now takes place in human bone marrow. B cells circulate around the body in the blood stream. When activated, they release huge amount of antibodies.

B cell production 

B cells start out as the same type of stem cell as the T cells. Instead of moving to the thymus, however, B cells move to bone marrow to mature. There they are given cell receptors, and are then released into the blood. Once released, they move to the lymphoid tissue of the body, where they are located nearby, but distinctly separate from, the T cells.

The production of B cells involves some incredibly complex genetics that is not worth speaking about here. Generally, imagine that the body has an enormous pile of building blocks with which to make something that will recognise a foreign substance. These building blocks can be placed together in millions of different combinations, and each will be able to recognise a certain substance. The body makes almost all possible combinations in the form of immunoglobulins and places them onto the surface of B cells, allowing them to recognise foreign substances.

B cell function

B cells play two major roles in the protection of the body:

• Ensuring antibody production against the appropriate target antigen; and
• Presenting antigens to T cells and providing signals for T cell activation.

B cell activation 

The majority of B cell activation takes place in the lymph nodes. Certain types of cells in the lymph nodes eat anything foreign and present them to B and T cells. Any B cell that shares a receptor for this substance will be activated and start to multiply. B cells can also be activated by helper T cells. After activation, active B cells migrate around the body and change into plasma cells.

Plasma cells

Plasma cells are B cells that remain committed to the production and secretion of a single antibody type. This secretion gives rise to the antibodies found in the circulation. Immunity is kept for as long as the plasma cell continues to secrete antibodies.  

Memory B cells

Memory B cells can also be formed after stimulation. These cells migrate to the lymph nodes, where they remain ready for further rounds of activation should the specific antigen ever be encountered again. If it is, then a very quick response can be made because memory B cells are ready and waiting to multiply.