Covid Explained: Simple Guide to Make Travel Decisions — Part 2


Ted Fahy and Priscilla Taylor are traveling with us to Chiapas in February 2022. We’ve been having back and forth conversations about traveling as safely as possible during COVID. Ted is a retired pediatric physician and Priscilla is a retired pediatric nurse practitioner. I invited them to write an occasional commentary about COVID, concentrating on its relevance to travel. This is Part 2. For Part 1, click here.

About Edward Fahy, MD and Priscilla Taylor, PNP

We are Ted, retired Pediatrician who is still an active Fellow of the American Academy of Pediatrics, and Priscilla, retired Pediatric Nurse Practitioner.  Between us we have 85 years of clinical training and experience.  Pediatrics has always been a medical discipline primarily concerned with infectious disease. Through long experience we are extremely familiar with infections.  Our goal is to explain the basics of epidemic disease so that you will have a context within which you can better evaluate information about COVID and better understand why at times there is so much confusion and lack of clarity.

A Primer on Infectious Diseases – Part II

ANTIGEN: Any substance (usually a sugar or protein) that can produce an immune response if the body identifies it as foreign.  For our purposes, antigens are on the surfaces of invading germs or on invaded human cells.

The Immune System

From birth, our body must identify any antigen that enters it as either “self” – or “non-self”.  It must do this for thousands of viruses and bacteria throughout our entire lives.  To accomplish this remarkable feat, there are two parts to the immune system:

  1. The Innate Immune System: Discovered 120 years ago when Metchnikoff saw white cells called phagocytes (“devouring cells”) ingest bacteria and kill them.   There are at least 5 different kinds of white cells reacting the moment we are attacked.  This system doesn’t recognize individual variants – it is generic, instantaneous, and life-long.  For some infections (e.g., strep and staph), it’s the only barrier we have.  Unfortunately, it doesn’t always work.  100 years ago people talked about friends who died of “blood poisoning” – staph or strep that entered the blood stream.  Until antibiotics, a deep splinter could be – and often was – fatal.
  • The Adaptive Immune System: The game changer in fighting infection.  It’s unique to vertebrates, can identify different germs, and remembers them.  It’s this memory that enables us to destroy a new chickenpox infection 20 years after our first bout; or to fight off pneumonia 20 years after we received the pneumonia vaccine.   It’s specific, takes weeks to fully mature, and can be life-long.

A clip about the two systems:  https://www.youtube.com/watch?v=yjAZXlMpw3k&t=9s

            There are two problems for the Adaptive Immune System to solve:

  1. How do you design a system that can notice that an antigen isn’t part of itself?  Ideally you want to produce an “attack” protein to fit the antigen – much like a key fits a lock.

Solution: Instead of creating a new key each time the body encounters a new lock, many billions of different keys are made in the fetus.  During gestation, so many keys are made that the odds are excellent that one of the keys will fit a presenting antigen.  Meanwhile, as the fetus grows, all the keys fitting the baby’s own antigens are destroyed. This keeps the baby’s immune system from attacking itself.  After birth, the remaining keys sit waiting for a bacterial or viral antigen to arrive.  When an antigen appears that doesn’t belong – BOOM!  The system is ready to respond.

  • How do you identify something foreign that is inside a human cell?  Bacteria float about outside our cells and are easy to find, but viruses and some bacteria are different.  They burrow inside our cells and are hidden, leaving only fragments of themselves on our cell surfaces.

Solution: Have two different “cell lines”.  One is expert at detecting free-floating antigens outside cells and one is expert at detecting viral/bacterial remnants on the surfaces of human cells that have been invaded.

These solutions are the basis of the two sub-branches of the Adaptive Immune System.  Branch 1 is the humoral immune system. It mostly uses “B cells”.  When a foreign antigen is found, B cells make millions of free-floating antibodies to the antigen so our body can destroy the germ.  Memory B cells are also made to recognize future attacks by that germ.  Branch 2 is the cellular immune system. It mostly uses “T cells”.  T cells are uniquely suited to “see” a remnant foreign antigen on the surface of our own cells.  When such an antigen is discovered, this system makes millions of T cells that specifically attack those antigens; and makes memory T cells to prevent future attacks.

The humoral system destroys both bacteria and viruses; the cellular system is more specialized and destroys viruses, fungi, and complex bacteria like tuberculosis. It can also kill cancer cells (which also have “foreign” antigens on the cell surface).

To fight COVID, we need both B-cells and T-cells in combination to detect the virus, to produce antibodies, and to have a robust cellular memory to hopefully eliminate the virus and reduce the risk of reinfection.

It is surprisingly difficult to find simple videos on these topics.  

Here are two complex teaching videos for the curious and courageous among you:

Humoral Immunity: https://www.youtube.com/watch?v=rAepZG_ChyQ

Cellular Immunity: https://www.youtube.com/watch?v=nqRn5fN22t4

Next: Epidemiology

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