Part I – Antibodies, Leukocytes & T Cells
Over the last several months we’ve learned a lot about the nature of the immune response when someone is sick with and then recovers from a SARS CoV2 infection. At first, it was unclear whether and for how long immunity might last after recovery. Most of the public and media discussion has been about the lasting effect of neutralizing antibodies following recovery from COVID-19…. but the immune system is so much more than just an antibody response.
This week I’m doing my best to explain the basics of the immune system, how it works and it’s different components.
What Is the Immune System?
The immune system is the body’s defense against infections from pathogens like bacteria and viruses. It consists of a combination of different kinds of cells that attack pathogens to keep us healthy. Our immune system can detect different kinds of disease agents and it’s really good at telling the difference between harmful and harmless or even beneficial organisms (e.g. viruses, bacteria, and parasites). It can tell the difference between the body’s own healthy cells or tissues, and ‘foreign’ cells.
When the immune system recognizes a pathogen (let’s take the SARS CoV2 virus as an example) it activates the immune system to protect the host. That immune response starts with intelligence gathering.
Intelligence Gathering
The immune system’s “intelligence gatherers” are called B lymphocytes. You might also know them as Bursa cells. These cells go around the body all the time and look for foreign objects that could be harmful. They’re looking for “antigens” or proteins that look strange and potentially harmful.
For example, the SARS CoV2 virus that causes COVID-19 has a has a protein coat that is unique to that virus. The antigens, or protein coat spikes are those sharp looking things that are often depicted in the media when they show the virus. The immune system’s B cells brush up against the virus and gather intelligence- and when they see the SARS CoV2 virus, they immediately think uh-oh. This is not good. Let’s make some antibodies to fight this antigen.
Emergency Response
Those antibodies are the “first responders” of the immune system. You can think of antibodies as firefighters that rush in to fight the threat that the invading virus may be posing. The B lymphocytes (B Cells) make the antibodies (also called immunoglobulins). They are proteins that lock onto the specific antigen that’s the threat.
You can think of it like a lock and key. The key needs to fit the lock to open the door. It’s like that, the antibody needs to fit the antigen to work too. When the key and the lock fit- the harmful virus becomes neutralized. Not dead, but neutralized.
There are several different kinds of antibodies. Two of the most important at IgM and IgG. Both are antibodies, but they have slightly different purposes. The IgM antibodies produced by the B cells form first, within the first couple days after infection. They do an OK job, but they aren’t super effective- but they can be made really fast. IgG antibodies take longer to make but do a better job once they get there.
Antibodies lock on to the threat and neutralize it, but they can’t kill the virus without help. The antibodies put handcuffs on the virus so that it’s held in abeyance until assistance arrives. The first help to arrive is a group of proteins called “complement”.
The complement arrives and sees that an antibody has locked on to the SARS virus. Complement triggers a chemical reaction that damages or kills the viruses, infected cells and other close by cells that are healthy. The complement is a pretty blunt immune system instrument because it kills both the virus, infected cells and other cells that aren’t infected.
But more help is on the way. They’re called “phagocytes”.
Calling in the Cavalry
Phagocytes are immune system cells that “eat” harmful foreign particles, bacteria, as well as dead or dying cells. These cells come to the rescue when they see that antibodies have been neutralizing harmful pathogens and causing a complement response. These “professional” phagocytes include a host of different white blood cells called neutrophils, monocytes, macrophages and dendritic cells.
These cells act to kill the invading organism, but it comes at a cost, because healthy host bystander cells are destroyed as the phagocytes react to the threat. The macrophages and neutrophils for example cause an inflammatory response that damages healthy host tissue. They destroy pathogens by engulfing them and subjecting them to toxic chemicals.
Sometimes those toxic chemicals are released into the environment causing the damage to healthy host cells. This process helps the body get rid of the harmful invaders but also causes inflammation to normal healthy cells that are bystanders.
Bringing in the Specialists
The immune system specialists are the T Cells. They provide an important role in controlling and shaping the overall long-term immune response. They use signaling proteins called cytokines to recruit to mount an immune response. Other kinds of T cells are called “helper cells” which indirectly kill foreign cells by attracting phagocytes.
The T cells learn to specifically target the harmful invaders and can attack those invading virus and bacteria in a targeted way that causes less collateral damage to the surrounding tissue. They can also recognize, target, and destroy things like cancer cells.
People spend their whole career studying T cells and their function in the immune system- so their work is complex and important.
Part II Tomorrow: Is there Lasting Immunity Post Infection?