Korva is a scroll of the internet’s best ideas.
Dendritic cells audition every helper T cell
Hunting for a match
When the first line of defense is not enough, the dendritic cell goes to work. It rips bacteria into tiny parts and covers itself in them, then leaves the battlefield and enters the highway that connects all your tissues with your lymph nodes, the immune headquarters. It is looking for a helper T cell, a sort of all-purpose commander within the immune army, and not just any helper T cell: one with the right weapon for the bacteria that infected the wound.
So it goes around and rubs itself against every helper T cell it meets. Most of them are not interested. After a few hours, something clicks: a helper T cell recognizes the bacteria parts, and the dendritic cell activates it.
Why a match exists
A match exists because the body carries billions of unique helper T cells, each with a weapon aimed at a different possible enemy. Once the right one is activated, it begins to clone itself over and over, one becoming two, two becoming four, until there are thousands of them.
How can billions of T cells cover every enemy?
Each T cell builds its receptor by randomly shuffling gene segments, a process called V(D)J recombination. Susumu Tonegawa won the 1987 Nobel Prize in Physiology or Medicine for discovering how gene rearrangement generates antibody diversity, the same principle that shapes T cell receptors. The shuffling can produce far more distinct receptors than the body has cells. Each individual T cell ends up with a single receptor shape.
Why don't T cells attack the body itself?
T cells mature in the thymus, an organ that sits above the heart. There, cells whose receptors react strongly to the body's own proteins are ordered to die, a step called negative selection. The large majority of developing T cells fail this screening and are eliminated. The thymus is largest in childhood and shrinks steadily after puberty.
Why do lymph nodes swell during infection?
Lymph nodes are the meeting places where dendritic cells present bacterial fragments to T cells. When the right T cell is activated and starts cloning itself, the node fills with dividing cells and expands. More cells are also recruited into the node from the blood, and lymph flow through it slows. The nodes shrink back to their usual size after the infection is cleared.
More from Kurzgesagt – In a Nutshell
