human biology - How does Humira work when injected into patients with rheumatoid arthritis?

Afaik. RA and psoriasis are probably caused by a malfunction of genes which regulate IL-23 production. IL-23 is required for Th17 cells to survive. Th17 cells produce a lot of things which result in inflammation and since TNF-α is a key mediator in inflammation and break down of joints, reducing the amount of available TNF-α reduces the inflammation.

(Th17 cells produce IL-17, IL-21, IL-22, IL-6 and TNF-α. Th17 cells are playing against Treg cells, which try to protect self cells. IL-17 promote osteoclastogenesis, and osteoclasts break down bone. According to serveral wikipedia articles IL-17 induces the production of other cytokines such as IL-6, IL-1β, TNF-α. IL-21 is required for sustained CD8+ T cell effector activity. IL-22 is a mediator of cellular inflammatory responses. IL-6 modulates the resistance of T cells against apoptosis, induces activation of T helper cells and promotes osteoclastogenesis. TNF-α is an endogenous pyrogen which is able to induce fever, apoptotic cell death, cachexia, inflammation and inhibits tumorigenesis and viral replication. IL-1β is a mediator of the inflammatory response, and it is involved in the regulation of apoptosis as well.)

Adalimumab is an immunoglobulin, which binds to TNF-α, and so it prevents it to bind TNF-α receptors and cause inflammation. After it is bound to its target you cannot reuse it, that's why you need daily doses of it. It is degraded by liver and kidney cells, or probably by phagocytes as well.

There are other TNF-α blocking pharmaceuticals as well. There are IL-23 inhibitors, one of them is called ustekinumab. It is an immunoglobulin too and it is used by treating psoriasis. By RA it is currently under clinical trial, but I guess it will work by RA as well. There is an IL-23 inhibitor which is not an immunoglobulin and can be taken orally, it is called apilimod. Sadly it failed the clinical trials, it had only mild effect. It probably does not reach the inflammation in sufficient amount. Maybe later there will be an effective delivery method and so a drug which can be taken orally.

There are alternative natural therapies: TNF-α inhibitors which you can take orally, for example γ-linolenic acid, ω−3 fatty acids, chondroitin sulfate, etc... These can be useful by mild cases of RA. Probably fever therapy works by mild cases as well. Afaik. cryotherapy has nice results in treating RA and its positive effect lasts much longer than the effects of any drugs, so if you aren't afraid of cold, that is a nice alternative therapy as well.