The Role of Antibodies in the Immune System: A Closer Look

[mktsgm]

TL;DR Summary

If antigens can initiate an immune response, why antibodies are needed?

I have a doubt.

It is said that antigens detected by macrophages and and Dendritic cells (by way of PAMPs and DAMPs) can initiate and mount an immune response, why antibodies need to be produced.

Ok, antibodies opsonize the intruders. But then even without antibodies, the innate immune system such as recruitment of phagocytes like neutrophils is already at work! Also antibodies do not directly kill the pathogens. I wonder what is the role of antibodies here.

Also, is complement system is stimulated by antibodies or is it other way around?

I am missing something. Kindly clarify.

 

[BillTre]

The innate immune system is an evolved system that defends the body from a variety of long term and not fast changing threats.
It may work on things like molecules that make up bacterial cell walls (most of these are slowly evolving).

In the never ending arms race between hosts and pathogens, defense mechanisms that change only slowly can only provide limited benefits. This can be generally protective, but many pathogens will evolve evasions for static defenses.
Rapidly reproducing organisms like bacterial (which can got through several generations in a day) can quickly evolve new traits, allowing them to evade innate defense systems.
The antibody based adaptive immune system overcomes these limitations because it can defend the body by targeting molecules that it has not been exposed to before and even new molecules that people have only recently been created in labs.

Antibodies, once bound to their antigen, slightly change their shape, in non-binding areas, as a result of binding.
This provides a signal for other molecular interactions (other molecules binding the bound antibody molecule), such as triggering the complement cascade or binding by receptors on other cells (like macrophages).
An antibody (or a bunch of antibodies), may bind something like a bacterial cell, but is probably not going to kill the bacteria by itself on the bacterial surface. However, it can trigger actions of other bodially defense systems (complement of engulfment and digestion by macrophages) to eliminate the bacteria.

In all these complex interactions, antibody molecules bound to their antigens, provide direction to the activities of other immune system components that provide immune protection.

 

[Laroxe]

Its worth remembering that all pathogens are not the same and the effectiveness of our different defences vary, an example might be in the way many organisms can prevent phagocytosis. In response to any infection our bodies engage a number of different defences and these include chemical defences from the very start, in fact many of these are instrumental in the more specific responses that follow. As an infection develops so does the nature of the response and an early effect is on the resources invested in the defence this is followed by a more specific activation of the responding immune cells. In TB for example their is a change from neutrophils to lymphocytes being attracted to the site of infection and phagocytosis is reduced. This is in fact a very specific type of response as the TB bacillus can live inside phagocytes. There are lots of potential variations in the types of specific chemical responses that become more refined over time, the sort of problems that need to be addressed include enhancing the effectiveness of other parts of the immune system (complementation) by increasing the number of immune cells, this can be quite specific, reducing the pathogens ability to evade other systems by making them more recognisable, making infected cells more recognisable, and lowering the threshold of tolerance of the immune cells. Again as things get more specific the immune system might develop ways of damaging a bacterial protective coating or making them clump together. In bacterial infections the immune system may need to develop ways to neutralize toxins they produce and some antibodies are quite capable of causing so much damage to the cell wall of a bacteria it can kill them directly.
In viral infections antibodies bind to the surface proteins, some of which will be essential for the virus to gain entry to a cell, the better the binding to that particular protein the more effective the antibody is, if the virus can't gain entry they simply die. With some viral infections antibodies can also interfere with the release of virus from infected cells.
Really its difficult to split the immune system into different parts like innate or adaptive, in reality it works as a highly integrated system that has its own memory systems that allows it to protect us over long periods of time. Because even early responses to infection vary depending on the nature of the threat its all pretty adaptive.

 

[docnet]

Below are illustrations of the roles of the different types of antibodies and the components of the immune system they are involved in. (from Janeways immunobiology)