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Phytohaemagglutinin (PHA, or phytohemagglutinin) is a lectin found in plants, especially beans. It is found in the highest concentrations in uncooked red kidney beans and white kidney beans (also known as cannellini),^[1] and it is also found in lower quantities in many other types of green beans and other common beans (Phaseolus vulgaris), as well as broad beans (Vicia faba) such as fava beans.^[2] It has a number of physiological effects and is used in medical research. In high doses it is a toxin.

Crystal structure of PHA-L (Protein Data Bank identifier 1FAT)

The lectin has a number of effects on cell metabolism: it induces mitosis, and affects the cell membrane in regard to transport and permeability to proteins. It agglutinates most mammalian red blood cell types.

As a toxin it can cause poisoning in monogastric animals, such as humans, through the consumption of raw or improperly prepared kidney beans. Measured in haemagglutinating units (hau) a raw red kidney bean can contain up to 70,000 hau. This can be reduced around 200-fold by correct cooking (boiling for at least ten minutes).^[3] Cooking at 80 Celsius can raise the available hau up to five-fold. The bean also contains α-amylase inhibitor.^{[citation needed]}

Poisoning can be induced from as few as five raw beans and symptoms occur within three hours, beginning with nausea then vomiting which can be severe and sustained (profuse), then diarrhea. Recovery occurs within four or five hours of onset, usually without the need for any medical intervention.

Medically it is used as a mitogen to trigger cell division in T-lymphocytes, and to activate latent HIV-1 from human peripheral lymphocytes.

PHA actually consists of two closely related proteins, called PHA-L and PHA-E. The letters E and L point to the fact that these proteins agglutinate Erythrocytes and Leukocytes, respectively.

• Hamelryck T, Dao-Thi M, Poortmans F, Chrispeels M, Wyns L, Loris R (1996). "The crystallographic structure of phytohemagglutinin-L". J Biol Chem 271 (34): 20479-85. PMID 8702788. 

• MeSH Phytohemagglutinins