Correct! Adaptive immunity, also known as acquired immunity, develops gradually over time in response to specific antigens (such as proteins, polysaccharides or molecular components of bacteria, viruses, or tumor cells) displayed on antigen presenting cells (APCs) and presented to B- and T-lymphocytes.

Why is this system slow to respond initially but faster with subsequent infections?

Although T-lymphoblasts (thymocytes) develop and mature in the thymus, they do not originate there.

• Thymocytes that arrive to the thymus are considered to be in the double-negative stage and do not posses a T-cell receptor (TCR), CD4, or CD8 glycoproteins until they proliferate and begin differentiation in the cortex.
• Eventually both CD4 and CD8 are expressed (double-positive stage).
• These naïve thymocytes are schooled before they are released into circulation, undergoing a two-stage selection process that begins in the cortex with positive (clonal) selection.

What events occur during positive selection?

If lymphocytes pass the positive selection test in the thymic cortex, they move onto their next test in the: