Adaptive (acquired/specific) Immune System

The adaptive immune system has evolved to protect the body from an ever-changing landscape of pathogens.

Two arms of the adaptive immune defense:

B cells (antibodies target pathogens)  &  T cells (target infected cells)  [slides from A.K. Chakraborty]

T cells recognize (self) cells harboring pathogen:

Cells internally process self and foreign proteins, cut them to short peptides (8-15 amino acids).

Theses peptides (antigens) are presented on the cellsurface, attached to the protein major histocompatibility complex (MHC).

T cell receptors (TCRs) recognize pathogen peptides by binding to them strongly.

T cell receptors (TCRs) & B cell receptors (BCRs) must be

Diverse: More than 108 distinct T-cell types to recognize an evolving landscape of pathogens

The combinatorics of V(D)J recombination (video1, video2)

can probably generate more than 1012 - 23 distinct combinations. 

Specific: to lock on specific pathogens  for immunological memory (vaccination)

Self-tolerant: to avoid auto-immune diseases