You are correct in knowing that both diseases are auto-immune, but neither is really a deficiency. Rather they are relatively rare events that trick the immune systems into seeing our own (self) proteins as foreign. You are also correct in knowing that T cells are involved. You may know that CD4 or Th cells are the ones that go down in AIDS (a true deficiency). You have enough of them, so that isn't the problem.
T cells come in 2 major varieties: CD4 or helper cells (Th) and CD8 or cytotoxic cells (Tc). Th cells are central to the immune response by 'selecting' or supporting (helping) the response to produce antibody- or cell-mediated responses. Antibody-mediated responses are what protect you from bacterial infections and cell-mediated responses are aimed to kill viral infected cells. Antibodies are produced by B cells (one for each foreign protein). Virus-infected cells signal to Tc cells and the Tc cells will cause the viral infected cell to die. Th cells support both of these responses once they begin by producing immune system hormones.
Among others, two major hormones in this are TNF-alpha and INF-beta. Other immune system cells get involved and contribute to this complicated cell crosstalk and produce these two hormones and others as well. Immune responses are regulated by the balance of the hormones. Chronic inflammation from auto-immune responses as in MS and psoriasis can respond by increasing or decreasing these hormones. We know that in MS, patients have an antibody, triggered by some as yet unknown event. Psoriasis, on the other hand, is a cell-mediated problem. Thus the problem involves controlling the types of responses you have. In psoriasis for example, one report suggests decreasing the Th cells supporting Tc cells in the lesions decreases the severity of lesions.
In MS, the problem is partly to stop the later events from happening when antibodies find their matching "foreign" antigen. The two drugs you mention work in opposite ways. Rebif is IFN-beta and works to decrease the lesions of MS, but will result in an increase in the activity of Tc cells. Enbrel will decrease TNF-alpha, but that will make the lesions of MS worse. Other medical conditions are also affected this way.
Pregnancy can make TB and lupus (another autoimmune disease) worse and for much the same reason. The immune system hormones that ensure that the fetus is not rejected as foreign reduce the control that the immune system does have on the response to TB or lupus. You will need some patience to work through medication changes with your doctor. Controlling MS is sometimes a long process of trials with various drugs to maximize the control for the individual.