We all have an army of highly specialised cells, a first line of defence against invading foreign material.

We are born with this Innate Immunity  non-specific barriers of a chemical, physical and microbial nature.

Invaders can be identified by particular proteins unique to each of them, known as antigens.

As we experience attacks from the world around us, our bodies develop the knowledge and ability to specifically target anything that manages to evade our innate defences. This is called Acquired Immunity and can take two forms:

Passive acquired immunity:
When immune products are attained from another source such as a vaccine or during breast-feeding.

Active acquired immunity:
When the individual creates their own new immune products in response to invasion.

In both cases there are two stages to the process: recognition and response. Of these, the systems ability to recognise particular antigens is perhaps the most impressive and the most important. Not only is it essential for the system to know the difference between itself and the outside world, it must also be able to distinguish between things outside that pose a threat and those that dont.

It is when this recognition breaks down that immune diseases occur.

The immune response

The response, the action the system takes once it has established that there is a threat to the body, is designed to achieve two objectives: 1) to kill, neutralise and remove the invader; and 2) to remember it and prepare for the next exposure.

When talking about the immune response we are usually referring to a combination of all of these processes, conducted by a number of different systems and mechanisms.

Our immune systems extend throughout our whole bodies, encompassing many organs and working closely with other bodily systems. The immune system itself comprises the bone marrow and thymus gland.

All lymphocytes are produced in the bone marrow. Roughly half will mature to become what are known as B cells, while the remainder will travel in the blood to the thymus gland for maturation to become T cells. It is these two types of cells that are primarily responsible for the bodys ability to recognise and respond to foreign intruders.

Humoral and cellular immunity

Humoral immunity is the part of the process responsible for recognition. B cells and various other immune mediators take on the role of labelling the antigens with proteins called antibodies and ensuring the rest of the system is made aware of their presence.

The cellular response attacks and destroys the labelled cells, which are then cleared up and removed from the body.

Cellular immunity can be either specific or non-specific.

Specific cellular immunity is a type of acquired immunity, where the system responds to specific antigens that it has encountered before. This type of immunity is primarily executed by T cells, the chief mediators of the whole response. These cells have primary roles in co-ordinating and controlling all other aspects of the system.

There are several types of T cell, classified according to their primary roles. Cytotoxic T cells are the killer T cells, the main ones involved in specific cellular immunity. These attack and destroy foreign or infected cells.

Many other types, including natural killer cells, macrophages and granulocytes, take part in non-specific cellular immunity. This is a type of innate immunity and has 3 important roles: attacking foreign or infected cells, providing a hostile environment by secretion of products such as histamine to prevent replication of foreign or infected cells, and clearing the area of debris after an attack.

There is much overlap between the humoral and cellular responses.  B cells and T cells work closely together to co-ordinate the overall response. Helper T cells oversee the humoral response by alerting B cells to produce antibodies, thus allowing the B cells to label the foreign material for recognition by cytotoxic T cells and cells involved in non-specific cellular immunity.

Throughout the whole response, the immune system is developing memory. Each mature B cell that is produced and is capable of recognising particular antigens can become either an active plasma B cell or a memory B cell. These memory cells are specific to the antigen that is attacking at the time and are stored ready for the next exposure. This makes the subsequent response faster and more effective.

In the case of an allergic reaction, the most prevalent feature is the overreaction of B cells leading to the heightened response of one type of granulocyte known as mast cells. These then produce excessive amounts of histamine, causing the symptoms typically associated with allergies.

19/05/2009