The response of B lymphocytes to a foreign antigen, clonal selection and…

The idea

Each B lymphocyte matures carrying one specific antibody on its cell-surface membrane. When a complementary antigen binds to that membrane-bound antibody, this is clonal selection: the antigen selects the B cell whose receptor fits it. The B cell takes the antigen in by endocytosis and presents it on its membrane. A helper T cell with a complementary receptor binds and releases cytokines, which activate the B cell. The activated B cell divides repeatedly by mitosis (clonal expansion), producing a clone of genetically identical cells. Most differentiate into plasma cells, which secrete identical monoclonal antibodies into the blood. The remainder become memory B cells, which circulate to provide long-term immunity.

What earns the marks

• Antigen binds to the complementary antibody or receptor on one specific B cell; this is clonal selection.
• B cell processes and presents the antigen and is activated by cytokines from a helper T cell.
• Clonal expansion follows: division by mitosis to produce a clone of identical cells.
• Plasma cells secrete antibodies that are identical, or monoclonal, and specific to that antigen.
• The remainder of the clone differentiates into memory B cells.
• Distinguish clonal selection (antigen binding to the specific B cell) from clonal expansion (mitotic division of that activated cell).

Common mistakes and how to put them right

• Saying the B cell makes a new antibody to fit the antigen.

  Put it right: The antigen selects a pre-existing complementary B cell.

• Using clonal selection and clonal expansion interchangeably.

  Put it right: Keep the two terms for the two distinct events.

• Writing that B cells divide by meiosis.

  Put it right: Mitosis produces identical plasma and memory cells.

• Stating that memory B cells secrete antibodies.

  Put it right: Memory B cells remain in circulation and respond rapidly on re-exposure.

• Attributing clonal selection to helper T-cell activation.

  Put it right: Clonal selection is antigen binding to the B cell; helper cytokines drive clonal expansion.

Exam tips

• Always specify that plasma cells secrete antibodies; do not state that memory cells or T cells produce them.
• Use sequencing verbs (binds, presents, is activated, divides, differentiates, secretes) so the order of events is unambiguous.