The main function of the immune system is to fight infectious agents such as viruses, bacteria, parasites and fungi. To get an idea of the enormous complexity of this task, first think about what the immune system should not do\u2014attack the body’s own self. Somehow, the system should be able to distinguish between ‘self and ‘non-self. Every time it sees a non-self cell, the immune system has to attack it.<\/p>\n
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Obviously, it needs to remember such foreigners and how they look to be ready to face the next attack from the same organism. The best way to think of the immune system is to view it as an army defending the nation. The problems of the real army like logistics, transportation, supplies, etc., are all present in the immune army too.<\/p>\n
The foot soldiers in the immune army are the white blood cells. There are two types of white blood cells\u2014 lymphocytes and monocytes. Lymphocytes are further classified as T cells and B cells which are formed in the bone marrow. The T cells mature in the thymus and the B cells mature in the bone marrow; hence, the unsurprising choice of the prefix ‘T’ or ‘B’. There are several types of T cells that perform different functions and have different designations like T helper, T suppressor, etc.<\/p>\n
The job of the lookout in the immune army goes to the monocytes. When an infectious agent invades the body, it is a macrophage3 (a type of monocyte) that recognizes the foreign particle. The macrophage presents the evidence to the T-helper cell. The macrophage also releases a substance called interleukin-1 that stimulates the activity of the T-helper cell. The T-helper cells now sound the metaphorical alarm by releasing interleukin-2.<\/p>\n
In response to the alarm the T cells begin to proliferate. The result of all this activity is that another type of white blood cells called cytotoxic-killer cells begin to proliferate and attack and destroy the infectious agent.4 In scientific jargon, the T cells cause cell-mediated immunity.<\/p>\n
The B cells fight the infection in a different manner from that described earlier. A simplified model of how the immune system fights off a virus using the B cells is given below. Any infectious agent that enters the body will eventually be taken up in the lymph system.<\/p>\n
This may happen soon after infection, or it may not happen until the invader has found a niche and begun to replicate. In one of your lymph nodes, the infectious agent will meet a macrophage. The macrophage will ingest the invader.<\/p>\n
Then the macrophage takes the invader apart, and displays the viral antigens on its surface for other immune cells to read.<\/p>\n
Antigens are proteins specific to each particular micro-organism. The antigens act as an identity card that allows our immune system to recognize invader organisms that need to be eliminated. After displaying the agent’s antigens, the macrophage will send out a message to a T-helper cell to read and recognize the antigens.<\/p>\n
Once the T cell has read the antigens, it will send out messages to activate the B cells, which will in turn come and read the antigens from the macrophage’s surface.<\/p>\n
The activated B cell will then produce millions of antibodies. The antibody is a protein that will bind with an antigen. Each antibody is unique and specific; for example, a polio antibody will only bind with a polio virus. We produce antibodies because, given the high concentration of infectious agent that is needed to cause a disease, our macrophages could not go after the invaders alone. However, antibodies will outnumber the invaders and will effectively help us get rid of them.<\/p>\n
How do the antibodies bind with the infectious agent? The antibody resembles the mirror image of the antigen (like a key and a lock), usually providing such a close fit that, if they bump into each other, the antibody will grab the antigen without letting it go. Once an antibody has locked with an invader, it will broadcast a signal that says, ‘eat me and whatever I have captured’. A macrophage will in turn get the message and will devour the antibody-antigen complex and rid the body of the infectious agent.<\/p>\n
To hark back to our original description of a nation’s army, the communication and logistics is a very critical component of success and the case of the immune system is no different. The system is distributed throughout the circulatory system and needs reliable mechanisms for communication. The immune system uses chemical messengers that are transported by the blood. We briefly mentioned some of them\u2014interleukin-1 and -2 and the B-cell growth factor.<\/p>\n
As you can expect, the system is far more complex than outlined in this brief overview. For example, there are at least six other interleukins with specialized functions. There are other classes of messengers like the interferons that activate the lymphocytes (an area we have not discussed).<\/p>\n
I can picture to myself the macrophages going about their daily duty. As they are patrolling the body, they see the boring liver cells. Ignore and move on. Ha! suspicious activity near the stomach. No, false alarm, it is only the boring stomach cells. A little further out there\u2014found an invader! Attack, send out the alarm…. What happens when something goes wrong with this process? Either the macrophages do not identify the invaders or worse yet, attack the body cells. The latter results in horrible autoimmune diseases\u2014rheumatoid arthritis (joints are attacked), juvenile diabetes (pancreatic cells are attacked), multiple sclerosis (nerve cells are attacked).<\/p>\n","protected":false},"excerpt":{"rendered":"