Q.Define an antigen, giving one example of a substance that can act as an antigen.

An antibody is a Y-shaped H2L2 protein: two heavy + two light chains joined by disulphide bonds, with two variable-region antigen-binding sites at the tips of the arms and a constant-region stem.

Structure of an ideal antibody molecule:

1. Four polypeptide chains: Every antibody is made of four chains — two identical heavy (H) chains (longer) and two identical light (L) chains (shorter). This is written as H2L2.

2. Shape: The chains are arranged in a Y-shape. The two light chains lie along the two arms; the two heavy chains run through the arms and the stem.

3. Disulphide bonds: The chains are linked to one another by disulphide (–S–S–) bonds.

4. Variable (V) and constant (C) regions: The tip of each arm has a variable region (its amino-acid sequence differs from antibody to antibody); together the variable regions of an H and an L chain form an antigen-binding site. So each antibody has two antigen-binding sites. The rest of the molecule is the constant region.

Simple diagram (Y-shape):

   V   V        <- variable regions (antigen-binding sites)
    \ /   <- light chains on the arms
 H---H         <- heavy chains
     |
     |   <- constant region (stem)

Antibody structure is a favourite diagram question in NCERT/CBSE Class 12 Biology ('Human Health and Disease') and in NEET Biology, and is identical in the WBCHSE Class 12 syllabus.

Agglutination = antibody + particulate (cell-bound) antigen → visible clumping; precipitation = antibody + soluble antigen → insoluble precipitate. Filariasis is caused by Wuchereria bancrofti.

i) Basic difference between agglutination and precipitation:

• Agglutination: The antigen is particulate — present on the surface of cells or particles (e.g. bacteria, red blood cells). When antibody binds these, the cells/particles cross-link into visible clumps (agglutinates).
• Precipitation: The antigen is soluble (dissolved). When antibody binds a soluble antigen, they form a large insoluble complex that settles out as a precipitate. So the key difference is the physical nature of the antigen — cell-bound/particulate (agglutination) versus soluble (precipitation).

ii) Causative agent of Filariasis: the filarial nematode (roundworm) Wuchereria bancrofti (and Wuchereria malayi), transmitted by Culex mosquitoes; it blocks lymphatic vessels causing elephantiasis.

Antigen–antibody reactions and vector-borne diseases such as filariasis are part of 'Human Health and Disease' in the NCERT/CBSE and WBCHSE Class 12 Biology curriculum.