G.lamblia trophozoites have two nuclei and are flagellated.
Reside in the duodenum rather than colon.
Adhere to the intestinal epithelial cell rather than invading the epithelium , thus causing diarrhoea rather than dysentery.

- Mode of action:  

The trophozoites adhere to the sugars on the intestinal epithelial cells via parasite lectin which is activated when cleaved by proteases. Tight contact is made between parasite and intestinal epithelial cell via sucker-like disc.
Giardia block nutrient absorption by covering the surface of the epithelial cells or by damaging the microvilli.

                       

- Histologic features:
- One or two duodenal punch biopsy specimens are adequate for diagnosis (site of colonization of organism may vary ).
- Diagnostic feature: Identification of sickle shaped organism attached to the surface or
free within the mucus layer.
- Biopsy features range from normal to abnormal appearance of the intestinal mucosa.
- Mucosal changes are minimal in most cases.
- In some cases here may be clubbing of villus and decreased villus-crypt ratio.
- Crypt hyperplasia and focal epithelial damage may be noted.
- An increased mononuclear infiltrate may be present in the lamina propria.
- There may be increased intraepithelial lymphocytes.
- The brush border of the absorptive cells may be irregular.
- Sometimes villi may be absent and resemble atrophic stage of gluten-induced enteropathy.
- In case of immunodeficient patients, the histopathologist should look for plasma cells in the lamina propria. Nodular follicular hypertrophy of the mucosal lymphoid tissue is associated with common variable immunodeficiency.  Crypt cell loss and apoptosis may be noted in HIV positive cases.

Abstract:

Giardiasis: a histologic analysis of 567 cases. Scand J Gastroenterol 1997;32: 48-51

Histologic detection of trophozoites of Giardia lamblia in the terminal ileum.Scand J Gastroenterol. 1995 Sep;30(9):905-8.

Human giardiasis. A morphometric study of duodenal biopsy specimens in relation to the trophozoite count in the duodenal aspirate.Arch Pathol Lab Med. 1994 Sep;118(9):891-4.

Giardiasis: analysis of histological changes in biopsy specimens of 80 patients. J Clin Pathol 1990; 43: 641-643