This one is specially for my nani!
The immune system has a lot of cells, molecules and signals involved. Let’s explore the cells and their functions briefly. So, we can mainly divide the immune system responses into two different categories:
- Innate immunity
- Adaptive immunity
As the name suggests, the innate immunity is the kind of immunity we already have. We can call it quick and non-specific. So, for example, if you have get an infection, the immune cells that are part of the innate immune system will quickly respond and we will see an effect (fever, inflammation etc)
The innate immune system has external barriers and internal barriers. The external barriers include the skin and mucous membranes. Internal barriers include white blood cells, certain enzymes and chemical mediators.
For example, if a pathogen wants to enter your body, the first mechanical barrier it will face before getting into the body is the skin. Next, if it wants to enter through the nose, your nose hair (cilia) along with mucus will trap the pathogen and other dust particles. If it still manages to get inside the body through a cut or something, there are scavenger cells that will eat them up. All of these are a part of the innate immune system. This is all a part of general immunity. Maybe we can use an analogy of the building security guard here.
But how do they recognise the pathogens? They use pattern recognition receptors which are present on the cell. These receptors, as the name suggests, recognise patterns on the pathogen called pathogen associated molecular patterns. The receptors also recognise damage associated molecular patterns which are present on our own cells when they are damaged. When these patterns are recognised, the pathogen or body cells are killed.
Adaptive immunity on the other hand is slow and specific. Now, if the innate immune system fails to contain and destroy the pathogen, the adaptive immune response sets in after 4-7 days (this is why it is slow). The adaptive immune cells will target the pathogen more specifically rather than using general defence mechanisms. Another important thing to note about the adaptive immune system is that it can remember who attacked your body and what kind of responses destroyed it. So, next time, if the same pathogen enters the body, you will not fall sick because the adaptive immune memory cells will destroy the pathogen before you even notice any sort of effects. Let’s use the analogy of a personal body guard here.
Adaptive immune cells recognise the pathogens using antigen presentation which I will discuss below.
As you can see in the image above, there are plenty of cells involved in the defending our body and they all have different functions! Let’s see what these cells do.
White blood cells can be divided into granulocytes and agranulocytes and these are further divided into different cell types:
- Neutrophils: These are the first ones to reach the site of infection (generally bacterial and fungal infection) and release toxins that kill the pathogen. They also recruit other important immune cells to the site of infection.
- Eosinophils: The generally work against parasitic diseases and in allergic reactions by releasing toxins and other chemicals.
- Basophils: These also work against parasites and in allergies by realising histamine which causes inflammation.
- Macrophages: These cells are phagocytic, which means, they eat up cells. They have an important role in cancers where they eat up cancer cells (not always) and also eat up foreign pathogens. They stimulate a variety of other cells and work in co-ordination with them. They work as “antigen presenting cells” where they eat up the pathogen, process it and present a part of it to the other immune cells.
- Dendritic cells: The cells get their name from their long projections called dendrites. These cells also work as antigen presenting cells and stimulate the adaptive immune cells.
- Natural killer cells: As you can se in figure 1, these are a part of both innate and adaptive immunity. They are activate by macrophages that release cytokines (chemical messengers) to attract them. These have a special role in viral infections and tumours.
- Mast cells: These are also derived from the myeloid progenitor but not myeloblast. They release histamine (so they are involved in allergic reactions) and heparin (removes blood clots) which results in inflammation and blood vessel dilation. They recruit macrophages and neutrophils. So they can cause wound healing and defend against pathogens as well.
The compliment system: This is a very interesting part of the innate immune system (but can be activated by the adaptive immune response). We are going to look at a complicated image but that’s only to see how complicated the whole process is.
All we need to understand here is, the complement system has different pathways that it follows to reach the main role of making a membrane attack complex (MAC) which is a group of proteins that attacks the pathogen. It is activated when it recognises specific antibodies attached to the pathogen or when it itself recognises the carbohydrates present on the pathogen. A series of peptide fragments get activated one-by-one and finally form the MAC which actually punches holes in the pathogen (fig 4) causing it to lose its function and die. This also opsonises (coats) the pathogen marking it for destruction. Complement system activation causes inflammation to attract other phagocytes and results in clearing of the damaged material by the phagocytes. Hence it “complements” the immune system by increasing the immune response, giving it the name of complement system.
Moving on to the adaptive immune cells:
We have two types of adaptive immunity:
- Humoral: mediated by B cells
- Cell mediated: mediated by T cells
Don’t worry we are not going to go overboard with the information. I will briefly go through the the cells and their function. Just to remind you, the following stuff happens after 4-7 days when the innate immunity has failed to contain the pathogen.
- B cells: These will work on extracellular pathogens (the pathogens which are not inside the cell and are circulating freely), examine the pathogens and make antibodies against them (I will discuss the specific mechanisms in another blog). These antibodies attach to the antigen (a specific protein part of the pathogen which is unique to different pathogens) of the pathogen and kill them. At the same time, the B cells differentiate into memory B cells and make some antibodies against this specific pathogen. This antibody keeps circulating in the body all the time and next time the same pathogen enters, it kills it immediately (this is why it is called specific).
- T helper cells: These cells work on the phagocytosed microbe presenting the macrophages. Remember we discussed about the macrophages presenting the antigen to the cells? They present it to T helper cells, which activate the macrophage itself to kill it. As the name suggests, they also help out other cells and activate T killer cells.
- T killer cells: These are don cells with guns and knives (just kidding). These basically work on intracellular pathogens. So, for examples, the viruses that have infected the cells are intracellular pathogens. These are different than those that are eaten up or phagocytosed. When the antigen is presented to them, they directly kill the cell that is infected (hence don cells).
So, that’s about it (not really, this stuff is really vast). This is the basic summary of the immune system. I hope this was informative!
Oh, and if you feel lonely at any point of time, remember, there are more than 35,000,000,000 (35 billion) immune cells fighting for your life!
Opentextbc.ca Anatomy of lymphatic and immune system.