These cells make antibody. Their antigen receptor is the antibody on their surface. If a B-cell is actively secreting antibody, it is called a plasma cell. These cells kill virus infected cells and tumor cells. Fig 4.27 shows an NK cell killing a tumor cell.
Platelets are active in blood clotting and they are rich in inflammatory mediators.
The secondary lymphoid organs are connected by both lymphatic vessels and blood vessels. Interstitial fluid which percolates thru the tissues is collected by the lymphatic vessels and filtered through the lymph nodes before being shunted back into the bloodstream. 4.8 shows a dissection of a mouse that had been recently injected IV with a carbon suspension. Note where the carbon has localized. The liver and the spleen are very dark and the lungs and intestines are noticeably grayer.