Immunity: an army inside you

A review of the army

In front of you are the seven main branches. Tap any one and it steps out of the line.

Believers did not invent the military words. This is how the textbooks describe immunity: killer cells, memory cells, telling “friend from foe”.

A splinter in the finger

Skin above: a solid wall. A blood vessel below, and eaters stand watch beside it. Break the wall and see what happens.

Green rods: bacteria. In good conditions a bacterium divides once every 20 minutes. Time on screen is sped up.

A key to a lock nobody has seen yet

This is an antibody. With its two “hands” it grabs an enemy, and only one kind: each enemy needs its own shape of grip. There are millions of enemies, and new ones appear every year. And you have only about 20 thousand genes.

The solution is an engineering one: DNA holds not a finished weapon but a set of parts. Each B cell cuts out one part of each kind and glues its own variant together.

The combinations alone come to 1.8 million. And at the joints, as they are glued, random letters are added on purpose, and the variants become more than a hundred billion. The weapon against the enemy is ready before the enemy appears.

The number of parts according to the textbook Janeway's Immunobiology. Susumu Tonegawa discovered the assembly, Nobel Prize 1987.

One out of millions

Each dot here is a B cell with its own single antibody. Nobody knows in advance which one will be needed.

Red dots: the virus. Gold: clones of the cell that fitted, and their antibodies. One second on screen is about one day.

A school that expels almost everyone

The receptors of T cells are assembled at random too. So some of them will fit your own cells. That is why every recruit sits two exams in the thymus: can he read the “pass” of a cell, and will he attack his own.

For the second exam the cells of the thymus show samples of proteins from the whole body: insulin, proteins of the eye, the skin, the nerves. Whoever lunges at them is ordered to destroy himself.

An army from which not one link can be removed

Break a single gene, and a child is born with no immunity fit for battle. David Vetter of Texas lived all his 12 years with this disease inside a sterile bubble.

Even a bacterium, a single cell, has an immunity of its own, CRISPR. Some proteins write a piece of the virus DNA into an archive, and another finds the enemy from that record and cuts. A record without a knife protects nothing, and a knife without a record does not know whom to cut.