Mast cells. Mast cells and basophils are generally thought to be the major sources of histamine and can themselves be modulated by histamine as they express H 1, H 2 and H 4 receptors, although how this varies between different mast-cell types is not yet clear.
3. The pill is to be taken soon after unprotected sex. It should be taken preferably within 24 hours and before 72 hours or else the pill may turn useless. 4. I-pill checks pregnancy by preventing ovulation or fertilisation or implantation of the fertilised egg. 5. I-pill is 95 per cent effective if taken in time.
1 It may also be helpful to get this prescription ahead of time or to have Plan B at home, so it can be quickly and easily available should you need it.  Swallow the pills in the first dose as soon as possible. These pills can be taken up to up to 120 hours (or 5 days) after having unprotected sex.