Cells of immunity


My biology lessons started with cells: the building blocks of life. About 200 types of cells make up a human body, but the number depends on how you define ‘type’. It is hard to measure, but a recent estimate was the human body is composed of some 30 trillion human cells.

One trillion is a thousand billion or a million million or 1 with 12 zeros: 1,000,000,000,000.  A simpler way to write it out is to use the power of exponents: 10^12 (ten to the power of twelve).  There are also the estimated 39 trillion (39*10^9; 39,000,000,000,000) bugs that are part of each healthy adult human body; slightly more than the estimated human cells!

There are skin cells, liver cells, brain cells and so so, each adapted to its purpose. Blood cells come in three varieties:  white, red and broken.  Platelets are the remnants of broken cells that help form blood clots in response to a cut.  Red Blood Cells (RBCs) carry oxygen from your lungs to every cell in the body. The picture  below shows that blood below is mostly RBCs; they are doughnut shaped to maximise the amount of oxygen that they carry.  The shape results from  a spherical cell whose nucleus has been removed.  The RBC is thus not like other cells in that it is without a nucleus, and serves to transport Haemoglobin.  This is a protein molecule with iron at its core that binds the oxygen molecule when it passes through the lung and then deposits it in the tissues.


sem_blood_cellsWhite Blood Cells (WBCs) are the main cells of immunity, in particular the lymphocytes.  Another type of WBC includes the macrophage (‘Big Eater’), a general-purpose soldier, that removes all kinds of cellular debris and foreign matter.  In contrast lymphocytes provide immunity to a specific organism.  We learn about two types of specific immunity: from antibodies (humoral) and cells (cellular), with  B and T lymphocytes the responsible cells, respectively.

The reality is that is a simplification of a very complex system, with multiple different cell types, proteins, and chemical messengers involved.  For our purpose of understanding vaccines, the key point is that it is easy to measure humoral immunity: test for antibodies in the blood.  But testing for cellular immunity is harder, , and is not usually needed.  And when tests of immunity are done, it is usually use humoral immunity that is tested: antibodies.  In some cases (such as measles), the antibody provides a reasonable, but not perfect indication of protection.  For pertussis (whooping cough), we can measure various antibodies to the different parts of the bug, but not sure what level of which means protection.

The specific immune system requires exposure to learn. With a virus like measles, once the immune system has been exposed to an infection the immune memory that develops prevents further infection.  We develop life-long immunity, once we have recovered from the initial illness.  A virus like influenza is constantly changing so that it can escape detection by the immune system and cause infections many times.

The development of immune memory involves the selection of B and T cells. Those cells that are most effective through a match between antibody – a protein that will ‘fit’, like a lock and key, an ‘antigen’.  The antigen is usually a specific part of a protein or other component.  The antibody can ‘lock on’ to the antigen with relative degrees of good fit.  Those cells that produce the best fitting ones to circulating viruses are naturally selected.  The best ones proliferate to produce both the immediate attack on the bug, and a small number are retained as memory cells.  These are the ones that remain long-term ready to be reactivated, as well as those that continue to produce antibodies.

So, now we are ready to understand how a vaccine works…


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