Re: How long does it take for white blood cells to ward off viruses?

In order to eradicate a potent viral infection, various arms of the immune army need to be marshaled. Most viruses produce tell-tale "danger" molecules that set off the "Toll-like receptor" pathways in antigen-presenting cells such as dendritic cells (DCs).

These cells in the periphery (for example the mucosal tissues of the nose and throat) become activated and turn up their antigen-processing machinery, enabling them to collect and chop up proteins and other biomolecules which will act as targets with which the antigen receptors of the lymphocytes (white blood cells of the adaptive immune response, such as T and B cells) will compete to produce the best fit.

The dendritic cells need to then migrate to the local "fortresses" where the T and B cells hang out, waiting to respond to signs of invasion that the scouting DCs will bring to their stronghold. These organs are called lymph nodes; tonsils are a prime example whose swelling and pain most people are familiar with from childhood nose/throat infections.

To bring in the antigen, activated DCs use the lymphatic system as "highways" between peripheral tissue and the lymph nodes. Lymph node swelling during an immune response comes from expansion of the cells within, as the T cells with the best fit multiply to contain the infection and the B cell follicles expand to produce specific "bullets" called antibodies to target the invading viruses.

Having established this chain of events, let us consider the timing. In most mammals, it takes about a day to get optimal migration of activated, antigen-bearing DCs to the lymph node, although for certain subsets, this could take up to 2-3 days. Once in the lymph node, expansion of the T cell and B cell armies can take from 2-4 days. Hence, by the time the immune army is up to full strength, commonly around 3-6 days from the original invasion by virus has elapsed. This is why it takes a likewise number of days to clear the common cold. Infections lasting longer than a week should be a cause for concern as the immune system may be overwhelmed and outside help in the form of drugs and antibiotics may be needed.