Class I major histocompatibility complex protein (left) and MHC class II (right)

Know thyself

How does the immune system know you’re you?

Would you be able to tell one of your own cells from someone else’s, or from a bacterial cell? Perhaps not. But your immune system can distinguish self from non-self. If it couldn’t, its attempts to deal with foreign substances would be directed at your own cells.

Among the many different proteins found on the cell surface are the major histocompatibility complex (MHC) proteins. MHC proteins indicate that your cells are self and should be left alone. Apart from identical twins, we each have our own unique set of MHC proteins. Even a mother and her baby do not share the same ones. Scientists recently discovered that, during pregnancy, certain signals that recruit immune cells are turned off in part of the placenta, helping to prevent the mother’s body from attacking her child.

The recognition of non-self markers, known as antigens, on foreign materials triggers a response from the non-specific (innate) immune system. Pieces of these materials are presented to cells of the specific (adaptive) immune system, and a more focused, long-lasting form of immunity is developed, in the form of antibodies that can recognise the antigens in the future (see ‘Present and correct’ for more).

Lead image:

Class I major histocompatibility complex protein (left) and MHC class II (right).

Protein Data Bank CC BY


Further reading

Downloadable resources

About this resource

This resource was first published in ‘Immune System’ in January 2015.

Immune System
Education levels:
16–19, Continuing professional development