The body’s second line of defence

If the pathogens are able to get past the first line of defence, for example, through a cut in your skin, and an infection develops, the second line of defence becomes active. Through a sequence of steps called the immune response, the immune system attacks these pathogens.

The second line of defence is a group of cells, tissues and organs that work together to protect the body. This is the immune system.

Cells

The cells involved are white blood cells (leukocytes), which seek out and destroy disease-causing organisms or substances.

There are different types of leukocytes. Each of these cell types has a specific function, but they all work together to protect you.

NeutrophilsThese cells primarily attack bacteria. They are dispensable, rather like pawns on a chessboard. They rush to the site of incoming bacteria to fight them, but are easily killed. Neutrophils only last a few days in the body (before they self-destruct), but our bone marrow produces more every day. Some bacteria avoid neutrophils by hiding inside cells.

T helper cells: These cells are like the bosses. They give instructions to other cells by producing signals. Each T helper cell only looks out for one type of pathogen. Many T helper cells are needed to watch for many different diseases or invaders.

Cytotoxic (killer) T cells: These are killer cells. They punch holes in the walls of the pathogen cell so that the contents ooze out.

Macrophages: Macrophage means ‘big eater’. These cells ‘eat’ (ingest) or clean up the mess of dead cells.

Dendritic cells: These cells are like the spies. They notice if there is an invader and then present evidence of the invader to T cells in the lymph nodes.

B cells: These produce antibodies, which lock onto the antigen of invading bacteria and immobilise them until the macrophage consumes them. Some B cells become memory cells after being activated by the presence of antigen. These cells are able to live for a long time and can respond quickly following a second exposure to the same antigen.

Suppressor T cells: When the infection is gone, the immune system needs to be calmed down (or the killer cells may keep killing). The suppressor T cells slow down or turn off the immune system to prevent damage to good cells.

The tissues and organs

The tissues and organs involved in the immune system are the lymphatic system, lymph nodes and lymph fluid. These all have specific functions:

• The lymphatic system is a system of thin tubes that runs throughout the body. These tubes are called lymph vessels. They contain lymph.
• Lymph is fluid in which white blood (immune) cells are found.
• Lymph nodes are small, round masses of tissue that are found in certain areas (such as the neck, groin and armpits). They filter bacteria and other foreign materials out of lymph and expose them to B and T cells and macrophages that can engulf them. These cells multiply in response to accumulation of such materials, which is why lymph nodes swell during infections.

These cells, tissues and organs together form the body’s second line of defence against pathogenic microorganisms. They are responsible for the body’s ability to fight off infections and stay healthy.

How it works

When a pathogen (disease-causing organism) invades the body, the neutrophils gather at the entry site and try to engulf it and destroy it. Should the invaders get past the neutrophils, several things may happen. The macrophages (big eaters) will be attracted by the death throes of the neutrophils. These cells attempt to engulf the invader, but they also send signals to other cells for help.

The dendritic cells, which are constantly scouting around (they can communicate with up to 200 other cells at once) may find the pathogen and take a piece to present to T helper cells that congregate in the lymph nodes in the body.

A T helper cell will recognise only one pathogen. If the T helper cell recognises the invader, it will immediately clone to increase numbers. The T helper cell then activates the appropriate immune cells for the attack.

If the invading organism is a virus, the T helper cell will signal the cytotoxic (killer) T cells to come to the rescue. These cells punch holes in the walls of invading virus-infected host cells, killing the cells and destroying the virus. B cells can also be activated to produce antibodies that will stick to free viruses, marking them out for macrophages to clean up.

If the invader is bacterial, B cells are usually drawn to the task. They squirt an antibody (IgG) onto the bacteria (a bit like squirting tomato sauce onto chips). This sticks everything together and makes it attractive to macrophages, which clear up the mess. If the bacteria enters the system through the gut, the antibodies released by the B cells are usually IgA antibodies. These antibodies acts by blocking the bacteria adhering to the gut cells, preventing infection.

After a few days of fighting off the pathogen, you will recover from your infection. At this stage, the immune system activates T suppressor cells. These cells send out ‘calm down’ signals, which quieten down the immune system.

Sometimes our immune system doesn’t turn off or doesn’t co-ordinate well (especially if the T helper cells that do the co-ordinating are attacked). Autoimmune diseases, asthma and allergies are often a result of an inappropriate immune response. Other times, the pathogen is able to replicate rapidly or has tricks (like hiding) and defeats the immune system. That’s when people need extra help from doctors.

Prior to reading this article you might want to read about The body's first line of defence.