11.1.3—Primary immune response sequence

A primary immune response is the specific response to a newly encountered non-self antigen. Antigen presentation selects lymphocytes with complementary receptors, which then divide and differentiate into cells that act against the pathogen or infected body cells.

1. A macrophage presents pathogen antigen fragments; antigens may also be displayed on a pathogen or an infected body cell.
2. The B- and T-lymphocytes with receptors complementary to the antigen are selected. This is clonal selection.
3. The selected lymphocytes divide by mitosis, producing many clones with the same antigen specificity. This is clonal expansion.
4. B-cell clones form plasma cells, which secrete antibodies complementary to the antigen. T-cell clones form T-helper cells that stimulate B-cell division and T-killer cells that attach to infected cells and kill them.
5. The antibody response and T-killer action help remove the pathogen or infected cells. The first response is slow because selection, expansion and differentiation take time.

The response is specific because only lymphocytes with complementary receptors are selected. B cells provide the plasma-cell and antibody branch, while T cells provide helper and infected-cell-killing branches. This card stops at the primary effector response; persistent memory and the secondary response are taught separately.

Clonal selection identifies the matching lymphocytes; clonal expansion increases their number. Plasma cells secrete antibodies, whereas T-killer cells kill infected body cells. Do not treat macrophage antigen presentation as antibody production or include memory-cell mechanisms in this primary-response card.