immune system
Meaning of immune system: Defense rests on telling self from everything else, and its recognized failures run in both directions, too little response and too much.
Definition of immune system
The immune system is the network of organs, cells and proteins that defends the body against infection and clears away damaged or abnormal cells. It includes the bone marrow and thymus, where its cells are made and trained, the spleen and lymph nodes, where they gather and meet what they are fighting, and the white blood cells and antibodies circulating everywhere else.
How do innate and adaptive immunity differ?
| Feature | Innate | Adaptive |
|---|---|---|
| Speed | Minutes to hours | Days on a first encounter |
| Target | Patterns shared by many microbes | One specific molecular target |
| Memory | Little or none | Long lasting, and faster on re-exposure |
| Main players | Barriers, swallowing cells, complement proteins | T cells, B cells and the antibodies they make |
The two arms are not separate systems. The innate response arrives first, contains the problem, and then hands fragments of the invader to the adaptive arm, which is what allows a precise immune response to be built at all. Cells made in the bone marrow supply both.
How does vaccination use this?
A vaccine presents the adaptive arm with a harmless version of a target: a killed or weakened organism, a purified fragment, or instructions for a cell to make that fragment itself. Memory cells form as though the real infection had occurred, so a later encounter is met by a response that is quicker and stronger than a first one would have been. This is also why some vaccines are given in more than one dose, since each exposure sharpens the response further, and why protection against some organisms fades as memory cells decline.
What goes wrong?
Because the system must separate the body's own cells from foreign or damaged ones, it can fail in more than one direction. It can do too little, as in immunodeficiency that is inherited or acquired through illness or medication, leaving ordinary organisms free to cause serious infection. It can react strongly to something harmless, which is what allergy is. Or the training that normally removes cells reacting against the body itself can break down, and the response turns inward as autoimmune disease. The machinery is identical in all three; what differs is what it is aimed at and how forcefully.
Used in a sentence
Laboratory testing was performed to evaluate the function of the patient's immune system.
Good to know
Innate and adaptive immunity work together but on different timescales, since the innate response begins within minutes while the adaptive response takes days to build and then leaves lasting memory.
Word origin
Where “immune system” comes from
Formed from Latin immunis, meaning exempt or free from burden, from in-, meaning not, and munus, meaning service or duty.