The Cells of Your Immune System.

As you would have learnt in the previous blog post (see here), our Immune system is a complex system that serves to defend and fight against infection. The Immune system is made up of lots of different types of white blood cells (WBCs) that all start off as Stem Cells. In the diagram below you can see that the WBCs either belong to the Innate Immune System (orange colour) or the Adaptive Immune System (pink colour). You will notice that Natural Killer (NK) cells are included as part of both the innate and adaptive immune systems, though it works primarily in the Innate Immune Response.

So, what do these cells do to help protect us? How does it all work?

Innate Immune System

All cells Myeloid cells (cells that belong to the Innate Immune System) begin as Myeloid progenitor cell. Essentially, it’s like a plain cell waiting to differentiate (turn into), a more specialised cell. Similarly, from here you have Myeloid blast cells which are ready to become one of 4 types of cells, Neutrophils, Basophils, Eosinophils or Monocytes.
Our Innate Immune System can also be referred to as our second line of defence, that springs into action when our first line of defence has been compromised. Our first line of defence includes our skin which makes up a literal physical barrier against infection. Secretions such as salvia, mucus and tears all contain substances that help to ward off bacteria and keep us protected and are considered to be part of the first line of defence.

When pathogens get past this first line of defence, the innate immune system cells are ready to fight! They do this by creating an inflammatory response, blood flows to the infected area and WBCs leak out into the infection site in order to fight.

Neutrophils

- Make up 60-70% of your white blood cells.

- They are your first responders to any infection.

- They destroy pathogens with the help of blood chemicals and proteins that have varying antibiotic properties. They fight against bacterial, viral and fungal infections.  

Monocytes

- Make up 3-8% of white blood cells.  

- Whilst they are not as quick to act as Neutrophils, when they come to fight infections, they come in larger amounts and kill in larger amounts.  

- They can differentiate into much larger cells called macrophages. The role of the macrophage is essentially to clean up the mess made by the other immune cells when they are killing off microbes, ensuring that the infection site is left clean.

Eosinophils

- Make up 2-4% of white blood cells.

- Eosinophils are special cells that work specifically against parasitic infections.   

- They also work in allergic type reactions.  

Basophils

- Make up <1 % of white blood cells.

- Basophils main job is to manage the short-term inflammatory response in allergic reactions. Basophils release histamine and other blood chemicals in order to help to body overcome the allergic reaction.

Mast Cells

- These cells are very similar to basophils; in that they help with the inflammatory response by releasing histamine.

- Mast cells are different form basophils because they are found in connective tissue and the skin, whereas basophils are found in the blood.

Natural Killer Cells

- Natural killer cells (NK cells), despite their name, don’t actually kill off infections. Once a host cell is infected by the pathogen, it will replicate and spread the infection. NK cells help to reduce the spread of infection by killing the host cell so that it cannot replicate and spread the infection.

Adaptive Immune System

When the Innate Immune system has fought as hard as it can and is unable to manage an infection, the Adaptive Immune System steps in to help out. The major differences between the innate and adaptive immune system response is specificity and memory. Our body’s cells carry a special markers identifying themselves as ‘self’ (i.e., part of the body). The Adaptive Immune System is able to recognise ‘non-self’ cells (i.e., pathogens) and specifically attack these ones, compared to the Innate Immune response that attacks based on generalised threat. When the Adaptive Immune System makes a mistake and attacks a ‘self’ cell, this is termed an autoimmune response. The Adaptive Immune Response also has a great memory. It can remember if it has encountered a pathogen before so the next time it comes across it, it remembers it and can work more efficiently to get rid of it the second time around.

B Cells – Antibody mediated Immunity

- Are a type of WBC classified as lymphocytes

- B cells react to antigens. Antigens are substances such as the bacteria or virus your body is trying to fight off. The B cell, ‘swallows’ an antigen and then produces specific antibodies to help fight that antigen.  

Memory B Cells

- Think of the Memory B cell like the library for your immune system. These memory B cells tell the immune system, ‘Hey, we have fought this one before’ when it is re-exposed an antigen it has previously been exposed to. This is how vaccines work, a minuscule dose of a virus is given so that the Immune system is given a chance to create memory cells for that virus so that it will be able to fight it ff without you getting signs and symptoms of an infection.

T Cells – Cell Mediated Immunity

Cytotoxic T cells

- Main role is to actively fight infection by releasing toxins and chemicals to kill specific antigens on infected cells.

Helper T Cell

- Helper T cell loves to help wherever it can! It will secrete chemicals to attract macrophages to help with the clean up or neutrophils to help with the fight.  

Memory T Cell

- Memory T cells are similar to Memory B cells in recognising previous infections. When they encounter and recognise previous infections, they can turn into cytotoxic T cells and kill it off.