Immunology studies the human defense mechanisms that respond to the presence of foreign particles. It also covers areas such as transplantation, immune deficiency, and autoimmune diseases.

There are two branches of the immune response: humoral and cell-mediated.

Humoral immunity involves the activity of B lymphocytes, antibodies, and related cytokines. B cells respond to the presence of antigens and differentiate into antibody-secreting plasma cells. Antibodies can neutralize antigens, activate phagocytic cells, or recruit lytic enzymes to destroy foreign cells. A percentage of B cells differentiate into memory cells, which are implicated in the stronger humoral response upon reexposure to the same antigen.

Cell-mediated immunity involves T cells and related cytokines. Unlike B cells, T cells require antigen presentation via the major histocompatibility complex (MHC) for activation.

The degree to which an organism accepts the organs, tissues, blood or cells from another individual, without eliciting an immune response.

The human leukocyte antigen is the human form of the major histocompatibility complex (MHC).