Question: How does a humans immune system defend viruses?

Ian Hands-Portman answered on 21 Mar 2014:

It’s very complicated! I don’t understand half of it but get yourself settled and I’ll do my best.

Roughly speaking we have two sides to our immune system –

The Innate Immune System which spots anything that’s not part of our body and tries to attack it. It includes cells like macrophages ( which means ‘big eater’ ) that just engulf anything they don’t recognise and try to digest it. Another big part of it is the Complement system ( don’t ask me who comes up with the names!) which is a whole cascade of different proteins that stick to anything unfamiliar in a special sequence which culminates in them trying to blow a hole in the side of whatever they’re attacking.

The innate system is great and does a good job but it doesn’t learn or remember any invaders it’s seen before – so we have another layer –

The adaptive immune system –

This is a whole selection of types of cell lumped under the title of ‘lymphocytes’ and split into two groups – T cells and B cells. We also call them white blood cells because they’re not red like our oxygen carrying blood cells.

Our cells have a sort of inspection system – they put pieces of whatever they’re doing onto their outside surface, if they pick up a virus then pieces of the viruses they’re making get put up there.

B cells go looking for these pieces and when they find them, pick them up, and display them so T cells can find them. When a T cell finds a B cell that’s showing piece of an invader ( we call them antigens ) then it sends a signal to the B cell make copies of itself and start churning out antibodies.

The antibodies are designed to stick to the invader and label it, anything labeled gets mopped up by our liver and our innate immune system.

B cells stick around after an infection has gone and make few antibodies now and then so if we see the same virus again we can fight it much more quickly – it’s why you don’t often get something like mumps twice and why vaccines work.

It’s complicated because it needs to be – by having so many parts and processes our immune system is set up so that it doesn’t go wrong too often and attack things it shouldn’t – when it does we get things like hayfever if our body sees pollen as an invader or even worse if it attacks our own cells, terrible autoimmune diseases like lupus.