Definition of APC: Any cell that can process and present antigenic peptides in association with class II MHC molecules and deliver a co-stimulatory signal necessary for T cell activation.
These cells possess class II MHC molecules on the surface.
APCs include macrophages (monocytes as blood macrophages and histiocytes as tissue macrophages), B cells and dendritic cells.
B cell as Antigen presenting cell
Step 1: B cell activation=Antigen binding to Naïve B cell receptor followed by Antigen degradation inside B cell
Step 2: B cell acts as antigen presenting cells (APCs). Degrades antigenic peptides are displayed on MHC Class II recetor.
-T cell receptor (TCR) of Th cell binds to MHC class II bound antigenic peptide
-TCR activates B cells by secreting chemokines such as interleukins.
Step 3: Clonal selection and Differentiation: That activated B cell divide continuously forming clone of B cells with that specificity.
Late these B cells differentiate into Plasma cells and memory B cells.
Step 4: Plasma cells produce antibodies and are released into the blood stream that binds to the antigen and neutralize it. Thus preventing further infection
-Memory B cells produced are responsible for immunologic memory. (when the same antigen attacks the body for the second time, memory B cells recognizes it quickly and trigger an enhanced immune response, thus preventing further infection).