Specificity and Cross reactivity

The attributes of host and parasite interactions that determine immune recognition are specificity and cross reactivity. The degree of differentiation between the varied antigens of immune system is measured by specificity. Whereas, cross-reactivity measures the extent to which varied antigens appear similar to the immune system. The infinitesimal attributes of specificity and cross-reactivity describes the process of selection and nature of antigenic variation that shape the distribution of modification in population.

The interaction between invading pathogen and T cells is as follows: cellular digestion of pathogen protein, transport of resulting peptides to endoplasmic reticulum, formation of MHC-peptide complex, and binding of MHC-peptide complex to TCR. It is known from recent studies that a TCR performs cross-reaction with ~105 varied peptides. When a TCR reacts with its specific peptide, the probability of reacting with a randomly chosen peptide is only ~10−4.  Hence, TCR is termed as highly cross-reactive and highly specific.