Question

Which Ig is produced in primary immune response:
(a) IgA
(b) IgE
(c) IgG
(d) IgM

Answer 1

Hint: An antibody is defined as a protein that is produced by B lymphocytes in response to the presence of an antigen. Antibody is gamma globulin in nature and it is also called Immunoglobulin (Ig). Immunoglobulins form $20\%$ of the total plasma proteins.

Complete answer:
Antibodies are proteins and collectively called immunoglobulins. Each immunoglobulin molecule consists of 4 polypeptide chains. There are two long chains called heavy chains and two short chains called light chains. Actually, each antibody has two halves, which are identical. The two halves are held together by a disulphide bond (S-S). The four polypeptide chains are held together to form a U-shaped molecule. The two tips of this Y-shaped molecule bind to the specific antigens between heavy chain and light chain in a lock-key fashion, forming an antigen-antibody complex. Each antigen has many different antigenic determinants, each of which matches a specific antibody and binds to it.
In humans, immunoglobulins are grouped into five classes:
IgA
IgD
IgE
IgG
IgM
The first option is IgA. It plays a role in localized defense mechanisms in external secretions like tears. It also prevents attachment of pathogens to epithelial cells.
The second option is IgE. On mast cells and basophils, when bound to antigens trigger the release of histamine for inflammation and allergic response.
The third option is IgG. It is the most abundant antibody in primary and secondary response, it crosses the placenta, and provides passive immunity to the foetus.
The fourth option is IgM. It is the first class of antibodies which is released by B cells during primary response.

So, the correct answer is ‘IgM’.

Note:
The most abundant antibody is IgG (about $80\%$ of all antibodies).
The antibody IgE is present in the least amount. It is secreted by plasma cells in skin and cells lining gastrointestinal and respiratory tracts.