When we think of immune system response, we tend to focus on the internal working of the immune system namely how our body produces antibodies and mobilise other immune cells like the natural killer cells to destroy the invaders.

However, we must remember that our first line of defense is our innate immunity that is our skin, nose, lungs and gut.

It’s only when the microbes get pass these that the second line of defense, immune system response is triggered.

Ok what happens when a microbe say a bacteria or a foreign particle manages to get pass the first line of defense?

Why does that area that is hurt eg a cut, scrape knee swells?

When we cut ourselves, the cut and the surrounding area becomes pink, hot and painful and very often swells too.

We call this inflammation and it serves to fence off the injured area from the undamaged cells.

The immune system response has been triggered.

A brief narrowing of the blood vessels near the injury site is followed by dilation.

As the blood rushes through, it causes the redness and the increase blood flow also generate heat.  

More immune cells are channelled to the affected area by this increased blood flow. 

At the same time, greater amounts of glucose and oxygen are available to nourish the cells of the inflamed tissue.

Macrophages and neutrophils migrate to the injured area and attach themselves to the sides and penetrates through the vessel walls.

These together with chemical substances that are released, causes the endothelial cells (capillaries) to contract and open the junctions between them wide to let fluid escape from the bloodstream into the tissues. 

This causes the swelling and pain. 

If for some reason the macrophages fails to get rid of the foreign particle or microorganism, the inflammation becomes chronic.

Microbes and Viruses

Microbes use different tactics to infect a person and therefore trigger different immune system response.

When the microbes or other antigens are detected by helper T cells which are always moving around in the blood and lymph, they trigger the B cells to become plasma cells which produce the antibodies.

These antibodies engulf the antigen and digest them but they are not capable of destroying them. This part of the immune system is also called “humoral immune system”, ie it only produces antibodies and neutralise the antigens.

Complement protein and phagocytes then rushes in to gobble them.

Antibodies can also neutralise toxins produced by different microorganisms and can also activate a group of 25 proteins called complement protein to assist in killing bacteria, viruses or infected cells.

These 25 proteins are manufactured in the liver and spleen.

When the first protein is activated, ie when it sees the fragments of the antigen on a MHC cell, each of the 25 component takes its turn in a precise chain of steps known as the “complement cascade”. 

The end result is a cylinder being inserted into the cell.

Fluids and molecules flows in and out causing the cells to swell and then burst.

These causes the blood vessels to dilate, contributes to the redness, swelling and pain of the infected area.

Viruses and parasites on the other hand must enter cells to survive.

These infected cells then uses its own MHC molecules to put fragments of the antigen on its surface which will then attract the cytotoxic T cell.

These T cells or killer cells are then called into action.

The T cells secretes lymphokines, which will recognise the taged antigens and destroy them.

Some lymphokines will encourage growth of more T cells, others will attract immune cells like the macrophage, granulocytes (neutrophils, eosinophils and basophils) and other lymphocytes to the site of infection, releases their stores of toxic chemicals and destroy the invader.

This is the other branch of the immune system known as “cell mediated immune system”.

The immune system is very complex and the above is a very simplistic view of immune system response.

Antibodies and killer cells and immune cells are used in a generic group.

In actual fact it involves so many different type of immune cells depending on the type of antigen and area of attack. 

Some are independent while others are interdependent.

Different enzymes and chemicals are released for each different attack.

There are also many technical names which is not the subject matter of this site.

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