The immune system protects the body by recognizing molecules that do not belong to it. The same skill that fights infection can attack a transplanted organ, and when it fails, cancer cells can escape.
Self and non-self
Almost every cell carries surface markers called HLA (human leukocyte antigens) that are nearly unique to each person. T cells check these markers and attack cells whose markers look foreign or abnormal.
Transplant rejection
A donor organ carries the donor's HLA markers, so the recipient's T cells and antibodies can attack it. Close matching of blood group and HLA lowers the risk, and lifelong immunosuppressive drugs keep the response in check.
Types of rejection
Hyperacute rejection happens within minutes from antibodies already present. Acute rejection appears over weeks to months, mostly driven by T cells, and chronic rejection slowly scars the organ over years.
Immune surveillance and cancer
Cytotoxic T cells and natural killer cells destroy many abnormal cells before they can grow. Tumors escape by hiding their markers or switching off T cells, which is why checkpoint inhibitor drugs that release this brake can treat some cancers.
The cost of suppression
Immunosuppressants, chemotherapy and some cancers lower the body's defenses. These patients get more infections, and their infections may show few of the usual signs.
At the bedside
In a transplant or chemotherapy patient, treat fever as an emergency: check the neutrophil count, start neutropenic precautions when needed and look for sepsis early. Inflammation may be blunted, so subtle signs matter.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).