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Innate and Adaptive Immunity

The innate immune system

 

The innate immune system is the first line of immune defense of our body. It is a non-specific immediate response without the need to remember a previous exposure but it does not confer a long-lasting immunity. It is present since birth but it has a limited and lower potency compared to adaptive immunity.

The innate immune system always attempts to immediately eliminate pathogens. It makes effort to control the infection until the adaptive immune system takes action. Upon repeated exposure to the same pathogens, its action remains the same unlike the adaptive immunity which acts faster and with more power on repeated exposure.

 

 

The main components of the innate immune system are:

1. physical barriers (skin)

2. phagocytic leukocytes

3. Dendritic cells

4. Natural killer (NK) cells

5. Plasma proteins of the complement system

 

Our first line of defense against foreign invaders is composed of the:

·Skin – The skin has a thick outer layer of dead cells that bacteria and viruses cannot penetrate on their own

·The epidermis secret acids in the sweat that help inhibit pathogens.

·Saliva and tears contain lysozyme which is an enzyme that destroys the cell membranes of some pathogens.

·The respiratory tract is lined with mucus to trap bacteria and dust. Furthermore, the cough reflex helps to get rid of bacteria that have advanced to inner respiratory tract.

·The digestive system has hydrochloric acid that can also kill vulnerable invaders.

 

The Second line of defense:

·Inflammatory response

·Phagocytosis

 

The inflammatory response

 

The inflammatory response is triggered by any damage in the tissues by a pathogen, a physical injury or an insect bite.

 

When the tissue is damaged or ruptured, histamines and cytokines are released by surrounding cells. Histamine (secreted by mast cells) dilates the blood vessels in the area to increase blood flow bringing in more red blood cells, white blood cells and blood plasma. Cytokines (secreted primarily by leukocytes) are cell signaling molecules that promote cell to cell communication in immune responses (humoral and cellular) and prompts movement of molecules including phagocytes towards the site of infection or trauma.

 

Significance of inflammatory response: It helps to clean and disinfect injured tissues. It also prevents the spread of pathogens by limiting them in the affected area (which in turn enhances phagocytosis). The blood plasma that leaks in the interstitial fluid will induce clotting and eventually, healing of the damaged tissues. Furthermore, inflammatory response also causes an increase in body temperature (fever) to help inhibit the reproduction of pathogens and also stimulate more phagocytes by increasing the production of leukocytes.

Results of inflammatory response: Redness, heat, swelling and pain. The increase in blood flow in the damaged area causes the tissues to become red and warm. The blood and the proteins produced cause the tissues to become swollen. The pain mediators increase in the area and because the tissues expand, they can cause pressure in the surrounding nerves, hence the pain. Normally, inflammatory response is limited only as long as the infection is there. The affected area should heal as soon as the pathogens as cleared out. Sometimes inflammation takes very long time to get controlled like in cases of degenerative inflammatory diseases, chronic inflammatory diseases or autoimmune inflammatory diseases. These uncontrolled cases result to damages in the body.

 

Phagocytosis

 

Phagocytosis is the process by which the white blood cells surround, engulf and destroy foreign substances.

The cells that carry out phagocytosis are collectively known as phagocytes which act like the soldiers of the immune system. Phagocytes eventually die and pus is formed from the dead tissue, dead bacteria as well as live and dead phagocytes.

Neutrophils and Macrophages are among the phagocytes.

Monocytes, a type of leukocyte, mature into macrophages when they are released in the bloodstream after detection of an invader. They are usually stationed in the tissues of the lungs, liver and lymph nodes where they stay for days or even years. Aside from phagocytosis, they also help in the formation of adaptive immunity. This is done after engulfing and killing the microbes, macrophages return to the lymph nodes to imprint the remnants of the invaders that they destroyed, thereby, creating a memory of which, so that the acquired or adaptive immunity is stimulated.

 

Dendritic cells

Dendritic cells are also capable of phagocytosis and contribute to the production of B cells and T cells. They are the most important APCs or antigen-presenting cells. APCs are mediators of immune responses by processing and presenting antigens to the T-cell receptors which in turn will stimulate the T cells.

Natural killer (NK) cells are small lymphocytes that induce apoptosis. After detection of infected cells, NK cells secrete a protein that will attach to them (for example, tumor cells or virus infected cells) then these cells will undergo autolysis before they can infect the other cells that are healthy.

 

 

Complement System

The complement system is composed of different plasma proteins. It enhances the ability of antibodies and phagocytes to do their duties. They bind to microbes so that phagocytosis becomes easier. They also attract production of more macrophage and neutrophils and help kill microbes by rupturing their cell membranes.

 

Adaptive Immunity

We have previously described the first and second line of defenses against foreign invaders. The third line of defense is the adaptive immune system.

The adaptive immune system is not present at birth but is acquired after contact with pathogens or foreign bodies. It prevents reinfection and the development is gradual. It is basically composed of the B-cells (bursa cells), T-cells (Thymus cells), antibody and the secondary lymphoid organs.

Adaptive or acquired immune response begins when an antibody detects a specific antigen. It has the ability to learn, adapt and remember. As the body is exposed repeatedly to the same antigen, adaptive immune response becomes faster and more efficient because it develops memory for the antigens.

Cells of the adaptive immune system:

Lymphocytes- They circulate in the bloodstream and lymphatic system and then later to where they are needed. There are 3 types: B cells, T cells and NK cells

B cells – They are formed in the bone marrow and their primary function is to make antibodies. Their surface is covered by thousands of receptors (BCRs/ B-cell receptors). B cells can also develop into plasma cells which produce antibodies.

Antibodies are also known as Immunoglobulins (Ig) that have the ability to detect a specific antigen, react to this antigen to help eliminate an infection.

T-cells are able to determine a cell that has been infected by a virus so it attacks the cell killing it along with the virus inside. They also secrete cytokines, the cell-signaling molecules (T-helper cells).

 

An overview of the adaptive immune response:

While the innate immune system is immediately doing its job during an attack of a foreign body or antigen, it also helps to stimulate adaptive immune response particularly the T cells and B cells, especially when the viruses are involved. We know very well that viruses are very tiny and they have the ability to mutate.

Adaptive immune response takes several days to develop. B cells and T cells have the ability to replicate or clone themselves which make them very efficient and powerful in containing the infection. The T cells signal the cytotoxic T cells and the NK cells to attack and kill the invaders. After the invaders are contained, the suppressor T cells signal the body that the threat is over and the fight must stop. In the event that the invaders are not stopped like in a case of viruses, B cells are called upon to produce a specific antigen against them. This is initiated by T helper cells. The antibody will attach itself to the antigens (invaders) and will send signal to other cells to further act on them.