If you suspect major deficiency of antibodies in a person, to which of the following would you look for confirmatory evidence ?

The old worn out $RBCs$ are filtered out by

Write a note on lymphoid organs.

Draw and describe the structure of antibody?

Consider the following statements

$I.$ People should get vaccination to avoid infection

$II.$ Vaccination is available against polio, cholera, typhoid, tuberculosis and many othe disease

$III.$ Eradication of vectors are necessary in diseases like malaria and filariasis

$IV. $ Dengue and chikengunya, both are spread by Culex mosquitoes

Which of the statements given above are correct?