'Pathogens' were discovered by

The main barrier which prevents the entry of disease-causing organisms into our body is

Lymphocytes secrete a protein which causes the dilation of blood vessels. The protein is

Assertion $A$: The spleen is a large reservoir of lymphocytes.
Reason $R$: The spleen acts as a filter of the blood by trapping blood-borne microorganisms.
Which option is correct for Assertion $A$ and Reason $R$?

Certain pathogens are tissue/organ specific. Justify the statement with suitable examples.

In $1943$,the causal organism and host of the Bengal famine was: