Hadrons can divided into two sub-groups, Baryons and Mesons. Examples of Baryons are Protons and Neutrons, and for Mesons, Kaon and Pion particles.
In most simple cases, the Baryons have a three-quark structure and the Mesons and a quark-antiquark structure. For 'antibaryons', this can be found by taking a three-antiquark structure.
For example, the proton (denoted p) has the quark structure uud (where u denotes the up quark with positive 23 charge and d denotes the down quark with negative 13 charge).
An important characteristic to remember is that Baryon number is conserved within an interaction. An ordinary Baryon has a Baryon number of 1 and ordinary quarks have a baryon number of 13, such that the uud combination gives 1 as the baryon number of a proton. Similarly, all antibaryons and antiquarks have the opposite baryon number value from their counterparts (using negatives where positives where) This is why an antiproton (denoted ¯p) has the quark structure ¯u¯u¯d, which if you notice is the complete opposite from the proton structure. The antiproton will therefore have the opposite baryon number (−1) and charge (−1).
Things get a bit strange with Mesons, such that some particles like the K0 is its own antiparticle. I'll explain how this can happen next:
Firstly, because Mesons have the quark-antiquark structure, it's baryon number is 0 (since recall that a quark has 13 and an antiquark has −13). We also need to introduce the strange quark, which has a charge of −13 and a baryon number of 13. (Look up strange number conservation, namely not occurring in weak interactions, for further extension here). Kaons have the superscript value of its charge such that K0 has a charge of 0. So we can 'make' it from the quark structure s¯u. Consider that the so-called antiparticle of this is going to be ¯su, but if we look at the charges this will also produce the K0, therefore the K0 is its own antiparticle. The antiparticle of K+ is K− too.
This answer is very long but hopefully it gives some insight into antiparticles within the hadron group.
