The Immune System

Many ask what it is that happens after you inject a vaccine. How exactly do vaccines work?

What is immunity and for how long does it last?

If babies get antibodies that protect against infectious diseases from a mothers breast milk then why do we need to vaccinate?

Let us try to answer these questions in the simplest manner possible. To do this we will need to divide the immune system into 3. We will call these the innate immune system, the adaptive immune system and the memory part of the immune system. The vaccine is injected either under the skin (subcutaneous e.g. Measles, Yellow fever), into the skin (intra-dermal e.g BCG vaccine) or into a muscle (intra-muscular e.g Hepatitis B, PCV, DT, Td, TT, DTwP, DTaP, Hib). Some vaccines like the Polio Vaccine and Rotavirus are given orally.

It may be of benefit to look at these 4 very brief posts before you carry on reading.

Basic definition of variolation: https://www.facebook.com/485383008656904/posts/818703875324814/?d=n

How the first vaccine was developed: https://www.facebook.com/485383008656904/posts/818728328655702/?d=n

A vaccine is specific to that particular germ: https://www.facebook.com/485383008656904/posts/818879151973953/?d=n

What are germs?: https://twitter.com/MrMkhatshane/status/1245475100855582725?s=20

Innate Immune System

This part of the immune system consists of cells that are basically like frontline soldiers. It is concerned with constant safeguarding and/or monitoring and has a response that is always ready to go.

• Macrophages: They detect the invader that is injected then start to release chemicals that will kill the invader. In this process they also signal the neutrophils. Macrophages will remain and fight the invader unlike neutrophils.
• Neutrophils: these are a certain type of white blood cell and account for 60-70% of all white blood cells. They get a signal from the macrophages then go to the target site where the vaccine was injected. They then help the macrophages with fighting the invader. However they are also called suicide soldiers because these cells die during this process.
• Dendritics: these cells do not fight at all. They get a good description of the pathogen then act as messengers and travel through our lymphatic system with the pathogen trying to get into the adaptive Immune System. The dendritic cell will continue along the lymph channel until it gets to a lymph node where it will meet the B cells and T cells.

Adaptive Immune System

Just a quick recap. The pathogen is introduced into the blood stream, the dendritic cells run towards the B & T cells while the neutrophils and macrophages stay and fight.

This part of the immune system is reserved for more targeted response. By this we mean that it will fight a specific pathogen with a specific weapon. We call these weapons antibodies.  These are specially trained soldiers depending on the message from the Innate Immune System. These cells are found in the lymph node.

• T-cells: there a different types of T-cells (CD4, CD8, helper T cells). Some signal (B-cells & macrophages) and some attack infections. Signaling means to pass a certain message to the other type of cell.
• B-cells : are larger than T-cells. They produce antibodies (immunoglobulins) and release them into the blood stream. The antibodies are specific to the certain pathogen that is infecting the body. After an Immune Response, a portion of the B-cells go back into the lymph node to become memory/plasma cells. These cells form a specialized/ targeted offense for that particular organism.

1. The dendritic cells pass the message to the T-cells.
2. The T-cells analyze the message then formulate an adequate code. This code is passed (signaling) on to the B-cells.
3. The B-cells release antibodies. When the antibodies run into an invader then KILL it. The antibodies attack and inactivate the virus/ bacteria/ fungus.
4. The T-cells will also go back to the site of infection through the lymphatic channel. They release cytokine inflammatory markers to hyper stimulate the macrophages (this is called the pro-inflammatory state).
5. Interferon gamma is a cytokine critical to both innate and adaptive immunity, and functions as the primary activator of macrophages, in addition to stimulating natural killer cells and neutrophils.
6. This causes macrophages to secrete additional (cytotoxic) chemicals to kill the bacteria or virus (pathogen).
7. T-cells also signal the innate immune system to release more macrophages.

Memory

After the B-cells have fought off the pathogen they become back-up soldiers (plasma memory cells). They remain in the immune system just in case that pathogen comes back. This saves time spent waiting on the T-cells to create a code that will be passed to the B-cells. The B-cells now already know what weapon to use and they just use that immediately without the invader causing disease. However, they still depend on the dendritic cells to present the antigen.

Important to note here though is that the memory cell is trained to fight one specific pathogen. It is ready to fight it off before it even establishes an infection. If a dendritic cell comes back (weeks/months/years) with a message that there’s the same organism then the B-cells can attack with the antibodies because they recognize the invader.

Each vaccine differs in how long it takes for a person to gain immunity and each vaccine differs on how long that immunity lasts for.

Some notes on the pro-inflammatory response

Inflammatory signalling molecules (Interleukin B, Interleukin 6 and TNF-alpha)

• an inflammatory cytokine or pro-inflammatory cytokine is a type of signaling molecule (a cytokine) that is secreted from immune cells like helper T cells (Th) and macrophages, and certain other cell types that promote inflammation.
• Inflammation is triggered when innate immune cells detect infection or tissue injury
• There are three main stages of inflammation which can each vary in intensity and duration:
  • Acute -swelling stage.
  • Sub-acute – regenerative stage.
  • Chronic – scar tissue maturation and re-modelling stage.

Take home message

1. Breastmilk does not have antibodies to all diseases. The immunity from breastmilk antibodies lasts up to 6 months.
2. It is essential to send children to the clinic for their scheduled immunizations.
3. A vaccine may need to be given more than once before a person gains immunity. These are called booster doses.
4. Live attenuated vaccines should be avoided or used with caution in people with weak immune systems because they may cause disease like symptoms.

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