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 You can buy these morning-after pills over the counter without a prescription in most drugstores and pharmacies.  These types of morning-after pills work best when you take them within 72 hours (3 days) after unprotected sex, but you can take them up to 5 days after. The sooner you take them, the better they work.