The third line of defence depends on lymphocytes. There are two basic types of lymphocyte and both are made in bone marrow.
One type, the T cells, mature after having first migrated from the bone marrow to the thymus gland. The other type, B cells, migrate to and then mature in either the bone marrow or in the foetal liver or spleen.
Once mature, they patrol around the blood and body, hunting for foreign antigens. T cells are involved in the cell-mediated response, whilst B cells are involved in the humoral response (both described below).
Different T cells have different receptor molecules on their surface. When an antigen invades the body, macrophages engulf it and present it to the lymphocytes.
If an antigen is presented to a T cell with a complementary shaped receptor, the T cell is stimulated, increases in size and starts to divide.
A clone of identical T cells is formed, all with the correct shaped receptor. These T cells then differentiate to form 4 groups of specialised T cells. These are:
- Killer T cells
- Helper T cells
- Suppressor T cells
- Memory cells
Members of this powerful infantry (except the memory cells) then make their way to the site of infection.
Killer T cells: combine with the antigens on the surface of any invading cell and release a powerful group of chemicals called lymphokines. Some lymphokines kill the pathogens directly, others stimulate other lymphocytes to become active, and still others increase the inflammation so that there are more macrophages.
Helper T cells: co-operate with B cells in antibody production (see later about antibodies). They also activate macrophages and promote inflammation.
Suppressor T cells: keep the immune system in check so that once the antigens have been dealt with, the system is switched off
Memory T cells: remain after the pathogens have been killed to stop re-infection (see lesson 4 on memory)
As with T cells, a B cell will form a clone if it comes into contact with a complementary shaped antigen. The clone contains mostly plasma cells for immediate use and some memory cells for use in the future.
The plasma cells are highly developed and are able to make several thousand antibody molecules every second.
Unlike the T cells, the B cells do not leave the lymph nodes - only the protein (antibody) molecules that they make move around the body. These proteins are released into the blood and carried to the area of infection.
They will be the right shape to bind with any appropriate antigen they meet but only the one that caused the stimulation of the B cell in the first place.
The antibody molecule, on the other hand, binds to the antigen in a similar way to a substrate binding with an enzyme. The fit, however is not as precise as the enzyme-substrate complex. The better the fit, the stronger the subsequent immune response will be.
By combining with the antigen labels the pathogen (which the antigen is attached to) as foreign. Often several antibodies combine with several antigens so that a complex mass is formed.
This action means that:
- The pathogens clumping together make them more vulnerable to phagocytes.
- The antibody "tags" the bacteria when it is stuck to it, making it more easily recognisable to phagocytes.
- Any antigens acting as toxins in your body are neutralised when the antibody sticks to it, i. e antibodies can act as antitoxins. In a similar way, if a virus has an antibody attached to it, it will no longer be able to attach or enter a host cell.
Unfortunately this defence system also means that recipients of organ transplants are not assured of an end to their troubles when this option is offered to save their lives.
It should be fairly obvious now that anything foreign in your body will be forcefully attacked. This means that an organ being transplanted from one person to another will be spotted as foreign (as it has different antigens) and could be destroyed.
The only way around this (unless you have an identical twin who can spare the appropriate part of the body) is to destroy the T-cells in your body using x-rays and immuno-suppressant drugs.
The downside is that with fewer T-cells patients are much more vulnerable to diseases that would not normally kill, e. g. pneumonia.