Antigen Presenting Cells APC

Are cells engulf antigen processing / fragmented it into small epitopes, process them then presented them on its surface,

To be vision by the immune system. they are called:

Professional APCs

Macrophages

Dendritic cells

B lymphocytes

Professional APCs are very efficient at internalizing antigen; by phagocytosis forming phagosome then lysosome gets contacted with phagosome containing antigen,

Secret proteolytic enzymes fragment antigen into small epitopes (each epitope of 6-8 A.A)

Every epitope then carried by specific MHC II and then displaying a fragment of the antigen, bound to a class II MHC molecule, on their membrane.

The T helper cell recognizes and interacts with the antigen-class II MHC molecule complex on the membrane of the antigen-presenting cell by both receptors:

T cell receptor (TCR) and CD4

T helper cell receptors can recognize from 1000 to 10,000 epitopes.

An additional co-stimulatory signal is then produced by the antigen-presenting cell, leading to activation of the T helper cell.

The expression of co-stimulatory molecules is a defining feature of professional APCs.