How do cells recognize each other?

Answer 1

Typically by surface antigens and other chemical markers. One can only wonder what they say to each other when they haven't seen each other for a long time ;-)

Answer 2

There are proteins called cell adhesion molecules located on the cell surface that enable one cell to recognize and bind to another cell. Examples include the cadherins; if cell A and cell B both have the same cadherin (e.g. VE cadherin), the cadherin molecules can interact and cause the cells to adhere to one another. Another example are cell adhesion molecules like ICAM and VCAM. These are present on the surface of the endothelial cells lining blood vessels in response to inflammation, and can interact with integrins (another cell surface protein) on the leukocytes in the blood to cause them to leave the blood and enter inflamed tissue.

Answer 3

yeah - that's the problem with cancer cells; they wear wrap-around sunglasses, and the immune system often doesn't recognize them.