Is a b cell lymphoma a particularly dangerous kind of cancer?

It depends. Left untreated any malignancy is ultimately fatal. There are different types of b cell lymphomas with different treatments and prognoses.
Lymphoma. Any cancer is a serious issue and it simply depends on the type of lymphoma as well as how advanced, the grading and staging, and if it has spread to other locations, and response to treatment. All very good questions for your oncologist.

Related Questions

If b-cell lymphoma is found in a breast biopsy, is that breast cancer?

Basically no. Common breast cancer derives from the glands and/or ducts of the breast. A few unusual sarcomas arise from the stroma of the breast. A lymphoma in the breast will behave like a lymphoma arising in the lymph nodes or wherever, and treatment will not be given as if it were a common breast cancer. Read more...
It is a cancer. in the breast, but it is not really a breast cancer. Breast cancer arises from tissue specific to the breast: ducts, lobules, and uncommonly supporting tissues. Lymphomas are cancers of the immune system that involve lymphoid tissue/lymphocytes, and can occur virtually anywhere in the body. Read more...