There is a correlation between the CD55, CD59, CD35 and CD46 expression on B lymphocytes of patients before and after treatment with rituximab and the level of depletion and repopulation time for these cells. The theoretical rationale of the study assumes that the correlation, if any, will be a negative correlation. However, the hypothesis of positive correlation (two-tailed test) will also be tested.

Rituximab, an anti-CD20 monoclonal antibody, is a new alternative treatment for patients with severe immune diseases and resistance to conventional treatment. One of the mechanisms of action of this drug is the complement-mediated lysis. Most RA patients respond well to rituximab treatment, however, some are refractory and mechanism of this non-response is still unclear. The role of CD55 and CD59 protein and its overexpression as a mechanism of resistance to rituximab treatment in lymphoma patients have been investigated. However, there has not been studied a correlation between the intensity of expression of these regulatory molecules on B cells of RA patients and resistance to treatment with Rituximab.

Inclusion Criteria:

1. Patients of both gender, aged greater than or equal to 18 years; and lower than 70 years.
2. Patients diagnosed with RA for at least 6 months according to the criteria of the American College of Rheumatology (ACR) in 1987 for classification of AR.
3. Patients with a medical indication for therapy with rituximab, with prescription of this therapy for the first time and have not yet received the first dose.
4. DAS28 score greater than or equal to 3.2.
5. Use of adequate contraception, as judged from the attending physician.
6. Desire to participate voluntarily, the ability to understand the protocol, documented by signing the consent form (attached).

Exclusion Criteria:

1. overlapping rheumatic disease, autoimmune lymphoproliferative and neoplastic.
2. active infection, patients with the use of cytotoxic drugs, presence of positive serology for HCV, HBV and HIV, active or latent tuberculosis.
3. allergy or hypersensitivity to rituximab.
4. pregnant woman or performing breastfeeding.
5. participating in another clinical study with intervention.
6. with functional class IV defined based on the criteria of Steinbrocker functional classification for RA.
7. prior therapy with Rituximab.

Patients diagnosed with RA and clinical indication for treatment with rituximab and who have obtained access to medication (private purchase, delivery by a private health plan, lawsuit or donation by the laboratory Roche) will be invited to participate in the study during the physician appointment.